The eladrin in Mordenkainen’s Tome of Foes are not in any way to be confused with the eladrin subrace of elves described in the Dungeon Master’s Guide. The Mordenkainen’s eladrin are CR 10 champions, each intimately associated with a different season of the year. Like many other fey creatures, they have a whimsical, fever-dream quality to their behavior: their decisions sometimes make more emotional sense than they do strategic sense.
One curious aspect of eladrin is that the four “types” aren’t separate beings at all. Eladrin morph from type to type according to the season—or their moods—with the metamorphosis taking place upon completing a long rest, so you don’t need to concern yourself with their changing type mid-encounter. They don’t do that.
What qualities do all the types of eladrin have in common? They all have resistance to physical damage from nonmagical attacks. They all have darkvision, ideal for the perpetual twilight of the Feywild. They all have superior natural armor, Magic Resistance, the Fey Step trait, and proficiency with both longswords and longbows. This constellation of features allows them to dart around a battlefield, engaging and disengaging as they please, suddenly appearing up close or far away—whichever is more inconvenient for their targets.
Spring eladrin are spellcasters first and skirmishers second, and they aren’t so much about slaying opponents as they are about shutting them down. The tricky part of managing their spell kit is that so many of their spells require concentration—and that so many affect only one target at once.
Enthrall and hallucinatory terrain are exceptions to both of these restrictions. Hallucinatory terrain has a long casting time, so it’s more for laying groundwork before a combat encounter takes place; the spring eladrin’s reasons for disguising the landscape may not even have anything to do with combat per se, only with a general intent to manipulate. Enthrall concentrates the attention of an entire group on the eladrin, allowing its allies to surround them or slip away, depending on what the situation calls for. But its range is the same as the range of the spring eladrin’s Joyful Presence, which charms creatures within a 60-foot radius.
This means that the other spells in the spring eladrin’s repertoire have to be used for dealing narrowly with those foes who are unaffected by both Joyful Presence and enthrall—and these are most likely to be ones who are resistant or immune to being charmed. Charm person, then, is obviously off the table except against a target on whom Joyful Presence has already worn off, or as a desperation move. Confusion requires concentration but is useful against multiple targets—four being the ideal number, per the “Targets in Area of Effect” table in chapter 8 of the DMG—if those who resist the spring eladrin’s charms happen to be appropriately clustered within a 10-foot-radius sphere. Tasha’s hideous laughter, because it incapacitates, is especially suitable against an individual of a martial class or subclass with Extra Attack. Suggestion and Otto’s irresistible dance are subpar for the spring eladrin’s purposes, although the latter can also be used against a target who’s shaken off Joyful Presence.
Belligerent opponents who resist its charms and insist on attacking the spring eladrin will find it to be an elusive target. When engaged by a melee attacker, it begins its turn with Fey Step to distance itself without provoking an opportunity attack, moves another 30 feet away, wheels about and Multiattacks with Longbow—once if it needs to cast a spell at the same time, twice if it doesn’t. When shot at by a ranged attacker, it does the opposite: moves as close to its foe as it can get, Fey Steps the remaining distance into melee reach and Multiattacks with Longsword, two-handed (since it carries no shield), once plus a spell or twice with no spell. If it can’t close the entire distance to its ranged-attacker enemy, it either casts an appropriate spell, Dodges (if it has allies present) or countersnipes with its Longbow. The spring eladrin is always thinking a move ahead, so you need to, too: if the belligerent foe whom the spring eladrin needs to deal with requires the use of the weapon it’s not wielding, give it a temporizing turn while Fey Step is on cooldown to stow one and draw the other. Player characters with their hands full may be willing to drop weapons on the ground to pull off short-term switches, but spring eladrin aren’t.
Also, since spring eladrin are less interested in winning a knock-down fight than they are in taking away their opponents’ ability to hurt them, you need to be conscious of why the combat encounter is taking place to begin with. Is the spring eladrin there to drive the PCs away? To prevent them from leaving? To steer them into a trap? To steer them past a danger? Is it working alone or with fey allies, and are they trying to harm the PCs or merely maneuvering them around as the spring eladrin is? Whatever this goal is, it’s always paramount, and the spring eladrin only attacks those who won’t be dissuaded from violent resistance; it would much prefer to handle the entire situation with Joyful Presence and its skill at Persuasion (or Deception). If it’s seriously wounded (reduced to 50 hp or fewer), it gives up, Dodging as it retreats.
The summer eladrin is an aggressive shock attacker, a flare-up of wrath personified, whose main objective—as can be inferred from Fearsome Presence—is to drive others off. If this trait doesn’t suffice to repel interlopers, the summer eladrin employs the edge of its sword instead.
When combat begins, its first move is to use its movement plus Fey Step to charge the nearest opponent who’s within 60 feet of all its other opponents, so that its Fearsome Presence can take effect, and use Multiattack to strike twice with Longsword, two-handed. The summer eladrin is less concerned than the spring eladrin with using Fey Step to disengage, because it’s counting on Fearsome Presence to get its enemies to run away from it rather than vice versa. As long as they’re frightened of it, they have disadvantage on attacks against it (including opportunity attacks), while it’s striking twice per turn and dealing an average of 20 damage with each hit, so they should want to move away—and it’s going to take its opportunity attack when they do. Of course, if it’s leaving a frightened melee opponent to go engage with a non-frightened one, it may as well use Fey Step at the start of that move to preclude the OA if the feature is available. But if Fey Step isn’t available, and it can reach that other opponent with its normal movement, it’s not going to worry about the OA.
On subsequent rounds, the summer eladrin chooses a target who’s overcome its Fearsome Presence or was never affected by it in the first place. If it can reach that target with normal movement alone or normal movement plus Fey Step, it Multiattacks with its Longsword. If it can’t, it stows its sword and takes out its bow while Fey Step is recharging, then Multiattacks with Longbow on its next turn. But if any enemy reverses direction and comes back toward it, it goes back to Longsword.
And if, when the PCs and the summer eladrin first meet, it’s more than 80 feet away from the nearest of them? Rather than charge a PC it can’t reach—and rather than lead with its bow—it uses its Intimidation skill to order them off. Combat ensues if they insist on approaching anyway.
A summer eladrin uses Parry against a normal melee attack only when there’s no other foe with a magic weapon that it may need to Parry instead. Parrying magic weapons always takes priority over Parrying nonmagical weapons. Because the summer eladrin is ruled by its temper, it doesn’t flee, no matter how badly wounded it is.
Autumn eladrin, like spring eladrin, are spellcasters first and foremost and are disinclined to fight—in fact, despite being chaotic neutral, they’re quite good-natured and interested in helping out. If possible, an autumn eladrin forestalls combat altogether by using its Insight skill to preemptively discern the PCs’ needs and “get to yes” with them. As it’s parleying, it calmly approaches until it’s within 60 feet of all of them, so that its Enchanting Presence will kick in immediately if combat should ensue.
Most of the autumn eladrin’s spells—cure wounds, lesser restoration, greater restoration, heal and raise dead—are boons rather than tactics. Sleep, as an innately cast 1st-level spell, is practically useless against a group of high-level adventurers; its only feasible application is as a gentle, nonlethal coup de grâce against one or maybe two already severely injured opponents. In a combat encounter, the autumn eladrin relies most heavily on calm emotions, which it uses to try to turn the hearts of those who insist on fighting it and who are unaffected by Enchanting Presence from hostility to indifference.
An autumn eladrin doesn’t move to attack. If a melee opponent comes to it, it attacks with Longsword—choosing to nonlethally knock its opponent unconscious if it reduces them to 0 hp—then Fey Steps away and uses the rest of its movement to get to a safe distance. It doesn’t resort to Longbow unless an intransigently antagonistic opponent resists its charms, in which case it moves to maintain a range of between 60 and 150 feet from that opponent and takes potshots until the quarrelsome idiot is finally either downed or dissuaded.
Foster Peace is an interesting feature, because it can’t be used to thwart an attack against the autumn eladrin itself—it affects creatures charmed by the eladrin, and part of the charmed condition is an inability to attack whoever or whatever has charmed you. It can therefore only be used by the autumn eladrin to thwart attacks against other creatures—allies of the eladrin, perhaps, or third-party entities—which may inspire a few ideas about what kinds of scenarios you might have an autumn eladrin show up in.
Winter eladrin are governed by melancholy; they fight, as they do everything else, with a sense of profound sadness that such an eventuality couldn’t have been avoided. They’re spellcasters, but not zippy-dodgy long-range spellcasters like spring eladrin. Winter eladrin don’t move unless they have to. They move their opponents instead, using gust of wind.
The winter eladrin’s weapon attacks do very little damage; they’re hardly worth using, even for opportunity attacks (Frigid Rebuke is a better use of the winter eladrin’s reaction). Its first line of defense is its Sorrowful Presence, but note that the DC of this feature is significantly lower than the Presence features of other eladrin. A higher-level adventurer who fails to beat it either has dumped Wisdom or is simply unlucky. Thus, it’s much more likely that a winter eladrin will have to resort to drastic measures to deal with aggressive interlopers.
These drastic measures are cone of cold and ice storm, each of which the winter eladrin gets to cast only once. Ice storm is for when four or more belligerents are conveniently clustered in a 20-foot-radius circle that doesn’t include any charmed target; cone of cold is reserved for when all the winter eladrin’s opponents insist on attacking it. In each case, just before casting, the winter eladrin uses Fey Step to reposition. Before casting cone of cold, it moves to the optimal place from which to freeze all its opponents in the blast. Before casting ice storm, it cheekily allows its foes to surround it—then Fey Steps away to a safe distance, centering the storm on where it used to be. (It’s immune to ice storm’s cold damage, but it would suffer the bludgeoning damage if it stayed put.)
While waiting for the preconditions of these spells to fall into place, the winter eladrin moodily stands pat, using Frigid Rebuke against whichever attacker seems likeliest to suffer the damage. Its Intelligence isn’t high enough to read Constitution modifiers off character sheets, but it is high enough for it to make comparisons by observation and judge which opponents are more or less hale than one another. Thus, on the first round of attacks, it may allow one or more opponents to hit it without consequence before invoking Frigid Rebuke. (This decision is essentially a “secretary problem,” in which out of n opponents, the winter eladrin allows n/e—rounded to the nearest whole number—to take swings at it unmolested before unleashing Frigid Rebuke on whichever subsequent opponent seems less tough than all who came before, or on the last one, if they’re the only one left.) After the first round, the winter eladrin has had the chance to compare all its assailants and can accurately judge which one has the lowest Con mod. But once another opponent manages to shake off Sorrowful Presence and joins the attack, it has to make the comparison all over again.
On its own turn, it aims gust of wind in whichever direction the spell pushes back the greatest number of enemies. In case of a tie, it aims it at the opponents with the lowest Strength modifiers—oh, yeah, while it was comparing everyone’s Con mods, it paid attention to their Strength mods, too.
Both fog cloud and gust of wind require concentration, but they don’t conflict, because the winter eladrin’s criteria for using them are wholly separate. Gust of wind is for repelling melee attackers while the winter eladrin stands its ground. Fog cloud is for covering the winter eladrin’s retreat when, upon being seriously wounded (reduced to 50 hp or fewer) or having spent its daily uses of both ice storm and cone of cold, it finds the idea of continuing to fight too depressing and decides to leave.
ETA an afterthought: My assumption above has been that each eladrin, since it doesn’t carry a shield, uses both its weapons two-handed, which necessitates stowing one while wielding the other, which takes time. It is possible, if you want to get more action out of an eladrin, to assume that it always carries its longbow in its off hand, wields its longsword one-handed in its main hand, and uses free interactions to draw the sword (when attacking with the sword) or sheathe the sword (when attacking with the bow) as needed, as it uses its main hand to nock arrows and draw the bow. This approach strikes me as somewhat more elegant from a game mechanics perspective but much, much less elegant from a biomechanics perspective: who does their best sword work while clutching a 6-foot bow in their other hand? (Yes, that’s how long longbows are. We treat them as the default bow because we like the damage and the range, but they really are enormous sons-of-guns. The same is true of longswords, TBH—when you think of a “sword,” generic, unless you grew up on Final Fantasy, you’re thinking of a shortsword or a rapier.) Anyway, I care a lot about verisimilitude, so I wouldn’t play it this way, but there’s nothing I’m aware of in RAW that says you can’t.