I tested an A5 merc with Dreadfang and i found the the Amp proc rate disappointing on the long term, even when you insert an Eth on the sword, to the point where the 7% Battle Cry proc rate interefers pretty often.
The Reaper + Blessed Aim combo feels significantly more reliable when jumping from pack to pack, but against big baddies without minions as well.
A Jah instead would most probably improve the proc rate against trashies, but then the merc's chance to hit would be even worse against bosses.
I honestly will try act1 merc with faith
Pros: you dont have to bother about ar
No cold dmg
You reach ias breakpoint for death cb easy (fuck grief)