My merc has a Hand of Justice runeword War Pike, which gives lvl 16 holy fire.
I guess that as up to act 3 hell he is killing most things in a single hit owing to the outrageous damage on the weapon, I haven't noticed the difference much yet
Once he's got one dead, a Corpse Explosion or two splats the pack. However because the weapon freezes enemies, everything but cold immunes just shatters - meaning no corpses for summoning or exploding. In this case, the holy fire helps counter the regeneration of the rest of the pack, which helps the skellies.
Holy fire is a good way to generate some great lag though
My clay golem just runs around like scrappy-doo shouting "
puddem up... lemme att'em"! He's very useful vs. bosses though!