I'd like to make a spell which lifts the target (like the Phonix's spell) but does not block its attacks, except against flying dudes. Ideas?

I would suggest modifying the ability to do this. A behaviour is used to keep the unit from attacking (among other things) so simply modifying this behaviour will allow units to attack, but since you want it to apply differently to ground and air units you will also have to modify the effects of the ability. I would suggest creating two different behaviours, one being the unmodified behaviour and the second being the modified one. To allow unit to attack set the Modification - Behaviours - State Flag - Suppress Attack to false. You will also need to add an apply behaviour effect for the modified behaviour. Now add validators onto both of the apply behaviour effects (one to only allow ground and the other to only allow air). Then add the newly created effect to the Anti Gravity (Periodic Set) effect.