I actually know many peoples who use merc and that manage to wipe out every encounter they do quite easyly, just train with your merc, use the right weapons and you should be able to do as well.Make encounter easier is not the way. Merc are quite enough powered to not give more advantage to them.
All you need is like 3 slaves and a rocket launcher and you do quite well.
If you want to make enemies, try to change something.
considering that using mercs as anything other than base defense is pissing your money away and slaves are just meat shields the easiest solution is to just only make players count towards increasing the encounter sizes.
I don't see slaves as meat shields really. If you arm them properly and use their combat advantages they can be quite lethal.