Actually for good crits u should have a archer Merc. And for the bossing better take a warrior merc with 100lvl and 10roubust and just keep him in for melee and archers take the boss like that.
Robust + Evasion are both maxed by level 120(merc) or something. so thats around the time mages can try epi 1 bossing(when u r 110 or so)
so HK mercs are good, coz they can tank bosses up front and also give good evasion