I don't avoid anyone or anything except Umbra. I don't go after her until I got that staff from Rindir's Staffs or I am a higher level.

Which brings me to the mobs I avoid at high levels if possible: Goblins.

In Fort Horunn, you can be mobbed by 6 or so Minotaur and Minotaur Lords, well Freazhder has. I avoid by chameleon when the odds are stacked against and move out of the dungeon. I try to avoid fighting wolves and weaker enemies (rats mudcrabs) as if they do not know you are there, they do little harm to the game. I avoid Scamps in Oblivion Gates for the same reason.
I am actively hunting Frost Titans.

Those [censored] mountain lions!! Theyre so fast and vicious... Being attacked by one is a near-certain deathsentence!! Ive lost many a stolen horse to their hungry jaws... And twice as many would-be adventurers came to ingolrious ends within the bowels of what is essentially the local wildlife!!

can't stand undead and special gloom wraiths are a pain in the ***

I have been having problems with zombies lately. For some reason they have become really hard to kill and seem to regenerate health at a substantial rate. It was so bad I thought the game was glitching. Maybe it is. But I use chameleon potions or invisibility potions to avoid them when possible now.

What level are you at? I think Dire Zombies... or Dread Zombies...something with a 'D' in any case... regenerate health. I recently had a tough run in with a pair of zombies... broke my weapon and my armour. They didnt regen though.