The strange fact is that even the server messages window didn't help, not telling me if that happened due to Spell Resistance (which I don't know if the critter has got) or an extreme AC (which I am sure he hasn't). To be sure I tried shot him like a billion times and in the event of SR at least one time I should have pierced it...
This issue happened to me once before, while adventuring with Syolkiir down the first cave in the Cloackwood forest. We have found a mindflyer that I manage to kill only thanks to my creepy chill tentacles (which is a spell like ability AoE). That time I thought the issue was related to the Spell Resistance of the mind flayer, but now I am starting being doubtful. Also becuase when a spell fails because of the SR the server window pops out its proper message (and both the times the server did not). Actually it popped a weird thing. I made a screenshot to help you understand.
http://imageshack.us/a/img22/7363/scree ... 4at152.jpg
Please help me if you can because for this reason already my mate has fallen even if we played well our excursion and.. Well.. I am scared to die to some random nasty critter