I'm over 20lvl mage and I find Lina really useful for him. Spamming Gizmo is just to much fun, I've got 2 turns recharge so It's nothing unusual seeing 2-3 Gizmos at the same time. With over 1-1.5k damage It's true power toy, only for something around 20 rage, even for mage that's nothing hard to accumulate. Especially when you combine with reaper 's rage draining (unfortunately It has long recharge, mostly I use it effectively once or twice per battle). Controlling Gizmos can be out off hand sometimes and It can spent it's valuable time on healing (with I personally didn't upgraded, just too weak), but if you place it behind lines It should do it's job (killing archers).
It looks like reaper's soul drain is also useful for mage, but personally I've got to greedy and went from 20% of stack kill for 5 rage to 30% for 20 and It got hard to use. I start battle with rage draining to get to 20rage so I've got mine soul draining blocked for several turns. Well I wish to stay with 5 rage. currently it would take something comparable to gizmos damage (~5k) to stronger foes but it seems like end game would be really heavy on big stacks, soul drain would get really hand then.
Also the orb creating skill of Lima is useful on the end of battle when you get low on mana for resurrection/sacrifice. Thought it's nothing big. Rest of reapers and lima stuff seems just to expensive.
Frog spirit seems mostly useful on the beginning and stone one is just useful for wasting unneeded rage. Later all you'll want to use is stone's wall (thought it needs some level ups for less rage use, it waste one units turn, not that bad yet 20 rage is really too much) and frog Armor (with actually is really useful, mine cost 10rage and considering its ablility to stop one attack (or counter attack) of foe no matter how strong it's something of thinking about - I suggest to get every possible upgrade of cost and recharge for it, It is worth it). Damaging skills are just to weak for big stacks off high HP units (with one exception - it's seems that, at least stone, damaging skills of spirits doesn't count as magic so it can damage black dragons, and trust me, those aren't easy to kill. A small nudge (worms style!
) over theirs grave sometimes seems reasonable).