Even if you have the perfect gear setup, the mercs only have 3 gear slots and will not be immune to being overwhelmed by monsters. Eth reaper's, fortitude/treachery and some elite helmet will make him be able to take on most monsters and packs, but not everything. Especially ranged monster packs.
Reaper's toll is to me the ultimate merc survival weapon, the decrepify slows enemies and make him leech more by dealing more damage.
But you have to reposition him and work with it, otherwise he will die eventually due to being overwhelmed.