There each of four rage spirits has only 4 abilities. Well, you can rise, until your max. rage, clearly, cause, if you raise, it's cost more, then you can't use him
About spirits abilities leveling more detail:
1.Zerock- suggest underground blades, it packs some dam. early, and later anyway you will more use other spirits.
2.Sleem- glot's armor are very useful, but not worth to lvl-up absorption.
3.Lina- chargers- should max., helps to recover mana, ice thorns- should max., for less cost, they can proptect your units quite well ; for rest- ice orb deals way more damage, if rolled, through field, but gizmo can add, or remove action points, choice of style.
4.Reaper- soul draining , try keep about 40% , if have enough rage, does insane dam.
; time back - should be as high, as your max. lvl unit, for use ; black hole- do not worth, for paladin ; rage drain - great combo with chargers
, should be high lvl.