Did you ever noticed the AI to destroy a Wall?
 

I mean the Wall casted by the first spirit?
This seems useless ability but providing the AI never breaks it it may be extremely usefull.

They break it, and it shatters completely by destroying just one piece, so it's far more useless than the wall of the ice spirit.

Depends on how you use it. If you make an area impassable, they will destroy it. If you place it in such a way that the enemy can get around but it'll waste a turn, they'll usually walk around it. If walking around the wall will take 2+ turns, they'll usually destroy it.

Something that has a price of zero and can absorb ONE enemy attack no matter how strong(one wasted attack), is tagged as 'useful' in my book :)

In my experience, there was not even a single time when I would have to or want to use it, it's not worth wasting your rage box for that turn.

I have used both walls on occasion, mainly when waiting for Reaper to finish resting. :)

I use Lina and Reaper in turns and the other two almost never get any action. On the odd chance that both Lina and Reaper have 1 round of rest left at the same time, I usually just use underground blades. Blocking the enemy can be easily done using phantom/summoning abilities/iceball/totems/demon portal/etc. and they all fare far better than any wall. :P

Yeah, but atm all i have are Zerock and Sleem, so i'm biased :)

Each of Lina, Reaper, and Sleem have two useful skills. On the other hand, the the rock spirit (name?), is pretty useless. I hope he gets boosted.

Underground rocks are pretty decent when maxed, puts some of the better spells to shame but yes, it loses it's usefulness in the second part of the game.