The Drunken Lizard Pub

Men im hitting it with fire dar at level 6 and nothing
Im mage level 12
all elemental powers
int 35 percp 40
elemental 33

Decided to post will try other spells now

deep freeze, fireball , supernova all nothing

Finally went nuts and hitted him with my nice dagger(attack 5) and killed it, is this the only way?

Yes, Poltergeists have the best resistances of any entity in Book I. They resist 100% of elemental attacks, in addition to toxins and disease. Pure magick attacks like Compress Atmosphere and Fleshboil will work 80% of the time, though.

What SpottedShroom said. Compress Atmosphere is about the only Elemental spell I've found to have much effect on poltergeists. They're the biggest challenge to trying to do a pure magic user who never attacks anything with a physical attack.