Ahsoka instakill

Yes

Ok i guess that means she isnt doing any damage, but instantly kills the enemy :D

That ability instantly defeats the target enemy and doesn't do any damage, so DI is moot.

Damage immunity doesn't stop an instakill from any character. There is a separate instakill prevention buff however, GL Rey applies it to her team for the first round for example.

Ben Solo’s “Force Dyad” unique does that, not Rey.