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.

Except that that uses Mana instead of Rage and it wastes a cast turn, which in alot of cases is more worth than a Chest turn. Not always useful but sometimes you just need to stop one stack from advancing.

That's true, a spell could come more in handy than a rage ability at times but making for instance a phantom and placing it within reach will more often than not soak a lot more than just one hit of damage, even more so if you make some thorns which cost no mana and are usually a high priority for enemy attacks. But I guess that's just from a warrior's perspective, I rely quite heavily on rage abilities and less on spells which are few and weak.

the AI sometimes destroys it, sometimes walks around. I find I very very rarely have use for it. But when I do it is a lifesaver.
In the right type of map, with natural choke points, I can use it to waste the turn of SEVERAL enemies. (by using them to block their comrades.

I believe that the AI logic in questions of walls is very direct/clear:

If there is a way around - they take it...

If AI has more units - it may calculate if it is more effective to lose one turn of one unit to destroy it - opening so the free way to others (I write "it may calculate" - as often I see that I would surely rather destroy the wall instead of going around with each of mine units...)

If the way around is longer that losing one turn on destroying the wall - AI will more positively destroy it...

Still I found that sometimes AI get stuck when trying to overcome the wall

If there is totally no way around the wall - the wall get's destroyed for 100%