So there are a number of exploits that require an enemy nearby, but for the case of armor or blocking skills you need to be getting hit. So I was curious. Of all the creatures in Skyrim, that will attack you, what does the lease damage? Per hit or Per minute.
I'm not interested in any solutions that require the use of the console or mods.
The least damaging creature is probably the humble Mudcrab.
That said, Armor and Block skills level up based on how much damage they prevent, not how many times they're used (this is the opposite of Oblivion's system), though the mudcrab should be just fine for leveling conjuration and alteration.
If you don't actually need to be hit (for instance, for Conjuration leveling with Bound Weapon spam, or Alteration): A Slaughterfish can be kept 'in combat' with you without ever actually being able to attack if you position yourself properly at the edge of the water.
If you do need to be hit (for armor skills, or block), a Mudcrab will do the least damage, but you'll level more effectively and efficiently by finding the strongest mob that you can sustainably heal yourself through getting beaten by. Defensive skill leveling is based on damage taken (or mitigated), not by attacks received.
I'd say it's probably a rabbit that would have been forced to attack you by using the "frenzy" spell on it and being the only target around.
