@MangaAddict69 You're right that wood will burn, but you overestimate how quickly the fire can burn through the wood. For instance, it would take around 20 minutes for a house fire to burn through a hard wooden door. That's a long time.
Considering that this is magical super fire vs magical super wood mecha nonsense, I'd say that even if it does get set on fire it won't be a problem until after the fight's over. Until then his wood robot gets +ATK bonuses from being a walking inferno.
@MangaAddict69
That wood isn't dry, to actually burn through live wood you need to evaporate the moisture inside first. Granted that he has mana flowing inside that wood, it adds another layer of strengthening to it.