You guys are hilarious. If you are not having mana issues.. an eth up'd shael'd kelpie snare is best or eth reaper shael'd if you cannot obtain kelpie. Obviously if you are having mana issues, a 17 meditation eth cv or eth ca are the acceptable weapon choice. The kelpie has slow and the reaper has decrep.
Now as for this silly argument about damage reduction. Mercenaries take 1000% damage from act bosses. That is from the arreat summit. If you put a shaft stop with 30% Physical Damage Reduction(PDR) on your mercenary.. he still takes 970% damage. SO, that being said, damage reduction of any kind is useless.
What is important?
RESISTS, ABSORB, SPEED, ARMOR, AURAS, LEVEL, LIFE LEECH, ABILITIES.
Resists : Max out your mercenary's resists. 75 is enough, but items that add max resist can make them even better, obviously.
Absorb : Same as resists.
Speed : The faster your mercenary attacks or recovers from hits, the faster he can move away, leech, deal damage.. etc.. it's self explanatory.
Armor : People preach high defense armor all the time for mercenaries. They do this for a reason! Mercenaries can wear really stat taxing items.. and eth items. This can result in huge levels of armor.. meaning they will rarely get hit. Top it off with appropriate levels and defiance aura... (assuming your a cold sorc). Just wow.
Auras : Important to you and them. The defiance aura is perfect for cold sorcs. If you are fire, holy freeze is best. And if you are a hybrid, it is up to you! Other auras are up for debate depending on what you are trying to accomplish.
Level : Your mercenarys' level is very important. If mephisto is lvl 87 in hell.. your mercenary will fair best starting at level 87 himself.
Life Leech : Your mercenary is doing a lot of physical damage and can benefit huge from having tons of life leech. Around 10% LL or more is good and will keep him alive.
Abilities : This is important. Having that kelpie or reaper will give your merc the ability to practically freeze enemies.. and it has the full affect on act bosses.. If your merc uses kelpie snare and blackhorns face.. they slow a boss down by 95%, attacks included. If I was that boss, I would be frantically save/exit when I saw how slow I was. It can be the biggest factor for a magic find sorceress to have a merc with kelpie/blackhorns on.
I hope this helps.. btw I know I didn't mention before, but Griz Caddy (armor) has around 1000defense and 3 open sockets. Socket 3 ptopaz in it and YES that will stack with your magic find, no matter who gets the kill shot.
One last thing to note is that you may be okay with non eth kelpie/reaper if you like to tele mephisto a lot. An eth reaper can easily 1 shot dolls in hell mode. They do a lot of damage, look it up! I would hate to see you insta-die from 1 or 2 of them exploding simply because your merc had that kind of damage.