act one rouge merc, it is good against zons and can tell you where danger might be if you dont have MH on. give it skullders, a shako, and a cruel hydra bow, and itll have lightning arrow, very good damage.
act 2 merc, it has good aruras, although i have found that its to eager to die...
act 3 merc, can have most possible MF for a merc, doesent melee attack as much as id like, insted uses spells to attack.
act 5 merc. makes a good meat sheild. very strong.
I personally would go with act one or two.