While poking around the game files for my own neferious purposes I have found the part which modifies the alien base stats for the different difficulty modes.
If you still want to nerf just the aim and crit values then take a look at DefaultGameCore.ini:
Starting around line 93 you will find the BalanceMods_... lines. There are a set for each difficultly level, and you should be able to change the modifications to the units here. I would try setting the iCritHit and iAim values to 0.
I haven't tried it yet, but that might do the trick.
Edit: After a couple of quick tries doing some simple stuff (shorter sat build time, etc) I found that just editing the DefaultGameCore.ini won't do it, as the game has them compiled into the exe for some reason.
The good news is that you can grab DarkReaver1980's AntiCheat mod, replace the DefaultGameCore.mod file with a copy of DefaultGameCore.ini, make your changes to that then run his modpatcher and it will then work.
Not quite simples, but almost.
Edited by Alan, 24 October 2012 - 09:29 PM.