There's a weaker respawning version of the Low-Demon outside its domain.
Monster A is monster A. All Minotaurs have the same stats, all Young Spider Queens have the same stats etcetera. The only thing that changes between bosses of the same type are the terrain effects(e.g. facing a Young Spider Queen in the dark or near torches) and the presence of allies(on either side, some bosses have adds).
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.
