Go to that map west of the Cloakwood Mines. There's nothing in that map except a cave with a Wyvern and lot of corpses inside. Kill the Wyvern (1400 xp) and rest, there's an almost 100% that another Wyvern will respawn. If it doesn't just rest again. A high level Blackguard can solo it real quick.
What about Keeper of the Portal, the guard outside Candlekeep?
I was using +2 weapons and spells with no effect, can he be taken down with poison / acid based attacks? Is he worth much XP?
Ngggh, he's unkillable, basically. You'll need something that deals Magic Damage (or directly a slay effect) and bypasses MR. Also, he only gives 1400 XP, which is the same as a Wyvern, and those are pretty easy to farm (see previous post).
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I was using +2 weapons and spells with no effect, can he be taken down with poison / acid based attacks? Is he worth much XP?