Wood woads are lawful neutral living plants, basically meaning, don’t start none, won’t be none. The only way you’re going to get in a fight with one is either to trespass on the territory it guards and initiate ruckus, or to attack it outright. Otherwise, they’re likely to remain indifferent to your presence. Not friendly—indifferent.
Wood woads are tough. They have 10 hit dice, exceptionally high Strength and Constitution, and a two-swing Multiattack with a Magic Club that does considerable whomp damage. However, they also have proficiency in Perception and Stealth—and advantage on Stealth checks in “terrain with ample obscuring plant life,” i.e., any kind of forest, wild or cultivated, or even a tall-grass prairie—so they won’t run straight at you as soon as they see you. Instead, they’ll blend in quietly, waiting to attack until trespassers come within reach—or, if they need to put an immediate end to a disturbance, closing the distance with Tree Stride, then attacking. Thus, their first attack will always be an ambush, with unseen-attacker advantage.
However, this ambush won’t necessarily be an attack with intent to harm. They’re lawful neutral, not lawful evil. They have proficiency in Athletics as well, and in combat, that usually means grappling or shoving. If they’re not outnumbered, rather than try to pummel trespassers, they may simply try to bounce them: grapple them, carry them to the edge of their territory and dump them there. Or they might shove a trespasser into a pit or trap, but chances are, it won’t be they who’ll dig that pit or build that trap. Shoving is a tactic they’ll generally use only when they’re henchmen of something or somebody else—though a roaming wood woad, if antagonized, might choose to shove an enemy into a ravine, if one happened to be nearby.
Being woody plants, wood woads are vulnerable to fire, which also inhibits their Regeneration feature. However, their fear of fire is offset by their devotion to duty, especially if they’re carrying that duty out. The result is that fire is more likely to anger them than deter them from doing their job. If a melee attacker strikes at a wood woad with a torch or a flaming magical weapon, the wood woad will use as many attacks as necessary to try to disarm its enemy (see “Action Options,” page 271 of the Dungeon Master’s Guide). If it succeeds, it will pick the weapon up itself as a free action after completing its Multiattack, and the next action it takes will be to hurl the weapon as far as it can from its owner. That will be at least 60 feet, possibly more, given that the wood woad has Strength 18 and isn’t aiming at anything. Only after all that will the wood woad resume attacking its foe directly, and that foe will get the sense that it’s pummeling him or her with particular ill will.
If the fire comes from a spell, that will attract the most unwelcome sort of attention from the wood woad toward the caster. It will close the distance by the fastest means possible, including Tree Stride if necessary, and focus all its attacks on that enemy until he or she is unconscious. (Regular readers may already be aware that I consider flaming arrows to be total bunkum and don’t allow player characters to use them in my campaigns.) If more than one enemy is attacking a wood woad (say, with a mix of fiery melee weapons and fiery spells), the wood woad will have to make a judgment call: Is it easier to disarm the melee opponent or to get to the caster and pound him or her insensible? The easier opponent to extinguish is the one the wood woad will go after first.
Being creatures of duty, wood woads will fight to the death in order to guard whatever they’ve been assigned to guard, but a roaming wood woad, encountered in a less structured setting, will retreat when seriously wounded (reduced to 30 hp or fewer), and if it’s being attacked with fire, it will retreat when moderately wounded (reduced to 52 hp or fewer). A retreating wood woad uses the Dodge action to impose disadvantage on incoming attacks and the Tree Stride feature to put as much distance as possible between its foes and itself.