You get to the various levels by:
1st - Retrieving the book that was stolen and returning it to Emelius the druid.
2nd - Killing the Ookwasps and getting Ulrik to complete the bridge.
3rd - Finding and giving a ring to Emelius the druid which he asks you for.
The learning tree quests are, to my knowledge, the only necessary quests to follow in
order to beat the game. (And by learning tree quests, I include quests given by others that complete the learning tree quests; ie, the elf's quests)
You must be able to access the 1st level of the learning tree in order to get the quest
to find the plains of the endless winds. I think you need access to level 2 in order
to learn about the ritual of sacrifices (gleaned by looking up the reference to Order
of Seasons) so that the elf will talk to you and send you on the quest to recover the
Milla'Kara shield. And you have to have access to level 3 in order to venture into
High Garendall to finish the game.
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Shoot 'em with everything you've got. If anything still lives, shoot 'em again.