Alright, I cooked this up/tested in about half an hour. What this mod does, basically, is change the mercy very slightly. As we all know, the mercy releases 8 mini missiles when it shoots, and these all track and kill your ACU. What this mod does it make their tracking a little worse, so that instead of mercies having pinpoint accuracy like a homing sniper rifle, they'll hit like a shotgun blast instead. End result being mercies are less effective against ACUs but more effective against groups of units.
Try it! Or, if you're like some people, don't try it and complain about it anyways.
Installation instructions: Place .zip in your My documents/my games/GPG/FA/Mods folder. That's it.
Edit: Version 2 is now up. The shots now cluster around the target, rather than behind it, mercies also have 100 hp (from 10) and have a turn rate of 1.0 (from 0.6). This should even them out a bit.
Also, I ran some quick tests. This is how the break-out worked:
10 mercs did about 9500 damage to an acu
1 merc killed 15/20 strikers (300 mass did 840 mass in damage)
2 mercs killed 5/20 pillars and damage 8 more (600 mass did over 990 mass in damage)
5 mercs killed 11/15 Loyalists and damaged the other 3 (1500 mass did over 5280 mass in damage)