IIRC 5 INT is drained with each hit. Therefore 3 hits can kill someone with less than 16 INT. Mages may survive four hits
I don't think there is anyway to avoid the INT drain when they hit you, althoug, their THACO is crap so as long as they don't stun u, u are all right. Use the cleric spell Chaotic Command to be immune to that psionic stun thing.