A (non-elixir) purple potion gives a temporary resistance boost (
Alchemy Spoiler Thread).
After completing the warrior guild tests you can learn a level of Boomerang. Did you try talking to / fighting the trainer again? I don't recall exactly how to get the skill, just testing it afterwards (I think I put another skill point in first, then tested it out on some lizards, and found that it didn't do damage fast enough to bother using; maybe when maxed it may have some reason to use over switching to a bow, etc).