Now that I've gotten the quest to deal with Whiteoak, I'm trying to kill it again, and it's still going poorly. What's worse, the guide specifically describes it as "pathetically easy", but it wrecks me with Tremor every time (I can't stop either it from casting Tremor or the saplings from weakening my magic defense with rocks)...
EDIT: I've noticed that the Whiteoak only casts Tremor if it gets Concentration up first, but since I go before it (even if I use Defensive Stance to cut my agility in half), I can't prevent this (the AI will choose to use Tremor and one-shot me with it even if I break concentration before it strikes).