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I've been playing for over a week or so, managed to conquer almost all the american continent. I have a nice array of human and alien weapons, and I'm being able to manufacture my own sun -alien- armor.

Things are going "balanced", as long as I don't run out of alien energy cells (Im mainly using alien weaponry). But there's a small problem with one of the muties lurking the earth:

There's the standard "Black Hawk Down" mission: A jet pilot stranded in no man's land, send your team to recover him, blah blah. So, my guys get in, share a few shots with some muties and then a mutie appears and paralyze all my squad.
I don't have time to return fire, I don't have chance to kill it, and it seems every single shot means freezing the target soldier. I do know there's a "psi" skill and I'm pretty sure it has something to do with my particular case and the resistance my guys have to that kind of attack, but the speed and good aim of the darned mutant makes me think that I'm forgetting something (Special armor, some tool, whatever).
If I have to test my luck against that kind of shots with the "psi" or "willpower" skill... well, I'm pretty much screwed :S

Any ideas¿
The best Advice I can give is travel in as large a group as possible and use weapons that pack a punch from a long distance (sniper type rifles are good) all fire at once and a single alien should go down almost instantly.
your stunned men will recover after a while.
worse alien weapons are to come, the rocket spitters are a nightmare,
I finally managed to take rid of those creatures with a small dose of luck. I use to go in full force (7 member squad) and in a "firing squad" fashion: As soon as I see one enemy, I try to make all my soldiers shoot him. Ammo ain't an issue yet.
The sun armor proved really useful to keep a nice degree of mobility while providing enough defense, too.

(I'm currently trying to break in the russian base, half of the world eaten by the biomass, and having a THOUGH time with the aliens with rocket launchers... :(
I never cluster my guys much because it makes rockets a nightmare.

But sniper rifles usually take out psi stunners and psi projectors at a good distance. Once you have some psi research under your belt, you also get a good anti-biomass option, and you get to use the stun-gun yourself.