Is there a mod that fixes the absurdly overpowered shield aliens from MZ? I just cant kill them...

Not to my knowledge, there are very few Zeta mods actually.

You can search on Nexus for Zeta mods, or check the MMM threads - it's possible that Martigen adjusted them.

Nexus is down

I have two mods that might help you without modifying the aliens themselves. My Microwave Emitter Fix and Deathclaw Gauntlet Fix restore the "ignore DR" attribute of those weapons that does not work in the base game. As a result, those weapons become very effective against highly armoured enemies. For example, a sneak crit from the ME will disintegrate any alien in one hit.

However, if I recall correctly that only worked in VATS. My mod works the same way as the Point Lookout Hillybillies. They also ignore armor. Due to the way this is set up skill does not matter anymore, but VATS is not needed. It needs FOSE though.

When Nexus is up again just search Gauntlet Fix. The one with my name in the title is surprisingly mine

Hmm, funny that the 'Ignore DR' bug is itself ignored for ages, then as soon as someone has the idea to fix it, another mod appears...

Anyway, my mods do indeed rely on VATS (unfortunately), but they also do take into account player skill and weapon condition when calculating the damage. They also adjust for critical/non-critical damage. Take your pick.

I made a small mod which makes the shield aliens only a bit stronger than normal aliens... Should I release it?

with WMK and MMM installed, I had no problems with the shielded aliens, one pickpocket and boom. silenced head shots from the sniper rifle also worked wonders