I think that "healthy" soldiers should be almost immune to psionic attacks. Only weakened soldier should be prone to PP. What is the point of PP, if you can't do anything to prevent it.
I suggest:
Consider soldier current state: bravery, injured, hp, overloaded, no friendly unit nearby, saw another soldier/civilian died, corpse is near, enemy units to friendly units ratio, low ammunition, distance to psionic alien, rested (fully rested only, if he wasn't on another mission in last X days)
Some resistance training would be nice. It could be really expensive and long. Maybe 5-20% resistance for 50-500K, one or two months long. This training or some device could also provide some hint where psionic alien is. Something like area, distance or direction.
Add another attack type to psionic alien for weakening soldier first. Soldier should be hit several times with this attack to be prone to other attacks.