Since fire is the main weapon for killing the white walkers. Won't it be a walk in the park for the dragons to annihilate the entire while walker army? I mean what can the white walkers even do against them?
Well the White Walkers themselves seem to be able to disrupt fire as seen when they attacked the Three Eyed Raven's cave. It'd be interesting to see how they fare against dragons. But yeah, the wights (undead army) seem to be afraid of fire, though I don't know if it's been established that they are weak to it.