I know the game is not very new, and I am aware this is not an important situation, but I find it a shame that these giants units make very low damages.

In my past game, one Urkan needed 8 turns to pillage one village !

I think Guardians and Urkans, should count as x6 units (like a stack of 6 units which is pillaging/sieging).

Nah. Gaurdians by their nature shouldn't be willing to pillage at all.