All Armor skills and block using... Slaughter fish!
Stand in water, equip armor type you want to raise, go afk.
I highly recommend you drink a health regeneration potion, as slaughter fish CAN hurt.
I will admit I have yet to test this, but it worked in other elder scrolls games, and in this one, armor has no durability... so afk seems fine. I have encountered slaughter fish in lakes, and they didn't do anything but startle me when they attacked. No real damage, but that could just be luck. I was wearing leather armor (fine) and had under 20 light armor skill at the time though, so I assume it is save as ever.
Confirmation will come tonight, unless someone else verifies before then. But I've used this in other ES games, and their damage in this one seemed low enough to do it in this one.