the following was taking from HALCIETS GUIDE located at the top of the barb forum!!
Act 2 mercs are your best bet, thanks to their good stats and auras, which help you and the rest of the party out a lot. There have traditionally been three big choices:
Act 2 Normal/Hell Defensive:
These guys give you the defiance aura, which can take your defense to insane proportions when combined with Shout and Concentrate. With decent gear and a high-level Shout from you, you can get this guy up around 20k defense, and yourself to 30k base, which makes both of you massive tanks.
Act 2 Nightmare Defensive:
Always popular for their Holy Freeze aura. Slower enemies mean less swings, less swings mean fewer hits. They were an excellent choice prior to the patch, but now, I would never hire one, for two reasons: one, some enemies cannot be frozen, and thus they are useless against them; and two, the “Doom†runeword. This thing is incredible. With it, any merc can be a holy freeze merc, thanks to the passive aura it provides. It also gives +2 to skills, making the merc’s personal aura that much stronger. Thus, there is really no reason in hiring a Holy Freeze merc, if you can get your hands on the runes for “Doomâ€.
Act 2 Nightmare Offensive:
Ah, the Might Merc, staple of the barb class. Regardless of the fact that his aura has been nerfed down a lot, it still tops out at level 20 by level 99, which gives a very respectable boost to your damage, and there are no enemies that ignore damage (except maybe an immune to physical, immune to magic unique that would nullify both concentrate and berserk). With good armor and your massive Shout, he’ll have no trouble surviving, and will greatly increase your killing speed. Throw on a “Doom†ethereal pole, and he’ll be kicking some crazy ***. Think about it: between this merc and your cries, you’ll have a health/mana boosting aura, a defense boosting aura, a skill boosting aura, a damage boosting aura, and holy freeze…my God.
So, in short, while the Defiance merc is nice, with your big defense, you won’t really need that much more. Go ahead and get some more damage from the Might merc.
Outfitting Your Merc:
Anything with big defense is a plus, and you should try and make sure everything he’s wearing is ethereal, since his gear doesn’t get damaged. He’ll need some life leech from something, but other than that, it’s up to you. Vampire Gaze and Shaftstop are nice for the pdr and leech. Crown of Thieves is very nice as well, and an “Honor†weapon is a powerful, east-to-make weapon for earlier in the game. I’d definately try and get the runes for “Doom,†just because that will make the merc much more powerful and useful. For the super rich, “Chains of Honor†is a nice choice for body armor, with the +2 skills (which will boost his aura) and life leech, and “Delirium†runeword is an incredible helm, with the % chance to cast Confuse. Doom, Delirium, and CoH, all in ethereal gear, would probably be the optimal equipment, though there are certainly many other options.