I also think Holy Freeze is the best, couple that with a Kelpie Snare/Reaper's Toll and most monsters a crippled. Holy freeze works against all types of monsters both ranged and melee since they attack slower, the only real downside is that a few monsters are immune to it's effects and that it doesn't work on act bosses.
i really wanted to "soupe" up my merc so i went doom, coh, eth rockstopper(?). i would have opted for might but didn't know if the +7k defense would seem more viable over the mercs boost in damage during pk. using a regular holy freeze merc is the next best way to go. gear him up with eth tolls reaper or eth bonehew, coh or eth stone, and some eth helm.
Damage Reduction and defence are more important than resists, when choosing armor, because the mercenary has very high inherent resists at high levels...So an ethereal upgraded Shaftstop would be a very cool armor and if your mercenary still hasn't maxed his resists from other equipment then socket an Um rune.
an eth item that becomes upgraded loses its eth defense bonus. don't do that. either reapers or bonehew w/ amn (eth preferred) for a weapon on holy freeze merc, or get a defiance merc and equip with doom poleaxe, which gives him holy freeze aura. a good armor for mercs in eth glads bane, CBF is priceless.