you need to learn then which
mercs to take and which
skill combination is most important.
Mercenaries lvl indicate nothing, you can have all mercs at lvl 29, with all epic equipment and you will loose; but u can have all mercs at lvl 25 with the RIGHT skills combination with only rare equipment and you will win without loosing even 1 merc.
its all about learning the game
.
I'll give you a small tip to start with
find 2 ghost stalkers with Dual wield + Critical skill, give them 1 throwing knife with high damage and and the other with high critical... (make sure 1 of the throwing knives has range +1), now go kill some enemies
, just a small tip in a sea of options
Good Luck