Hypno, nice Alchemy

It's easy to make many damaging potions. I like to creat 8 or so of the same kind. When you get in a REAL tough fight, hotkey the potion, then apply it several times. It will usually do something like dmg health 10 pts for 10 sec - hit 'em a few times with that and its all over.
An alternative to beating a tough boss is to combine enchantments/spells/potions for 100% chameleon and walk up and whack 'em. Umbra will not even be a challenge then (which spoils it a bit, but hey, if you're stuck...).