It drops the material Ancient Wood, one of the most valuable (it's worth 4000 G) enhancement mats in-game. As for beating Whiteoak, generally speaking you need to be able to reliably kill all the saplings in 1-2 turns. Once all the saplings are down it usually gets caught in a loop of reviving saplings and occasionally healing itself. At that point, as long as you can get through its passive healing - or god forbid, wait out its mana pool (you're looking at very close to 200 turns of skill use) - it's not all that terrible as a boss compared to other late game fights.
