This cheat works for Aveline and "Warrior class" Hawke. All you need is a shield.

It doesn't matter what kind however the cheat works better if you're using the strongest shield available.

All you have to do is equip the shield then un-equip the shield and the armor stat will keep going up.
There is a percentage next to the armor stat and when it reaches 100% then you or Aveline will no longer take damage.