Spirits of Rage

[The Rider]

How do they gain experience? At the beggining I thought they gain 1 exp point for every killed opponent, than I thought it depends on the level of rage skill they use and the number of uses of that skill per battle (It seems that on the first use of zerok's stone sword he gains 6 exp, on the second use 5 exp, on the third even less), the same is with the skills of sleem (I have only the first two spirits activated in the box).

How does the level of the spirits relate to the level of the hero. My hero is 15 lvl at the moment, Zerok is 10th level, Sleem is 12lvl. I think they are weak (and I used them in a strange system with three stacks of inquisitors - to raise my rage points, than I used many times the (heaven's) gift spell to activate again and again their resurrection ability and after all long battles with no losses (I used that system against shooters only, because I can control how long the battle will be that way) the rage spirits seems to be still weak. Is there a particular way for them gaining levels faster?