ethereal rare 511% ed Colossus Crossbow is about all that makes it powerful...
for head and body you want to have 4 open sockets in each to throw uber emeralds into for +240 dexterity (a Holy-crafted armor is very good to have in many situations, though imo having 2 armors handy is nice, I prefer to have a 4-emerald Tyrael along with a 4-emerald Holy armor)
There are a lot of ways to go about this, because that's about all you need to make or break your merc--the rest is just filling in the holes (leech, life per kill, resistances, DR if you choose). Hopefully HolyGhost will post his A1 merc's gear for you.
(good luck getting that shit)For the sake of not leaving you hanging, I'll post my necro's A1 merc gear at the time being, though there are better combinations if you're insanely rich.
Weapon - Ethereal Rare 511% ed Colossus Crossbow (4 sockets + jewels)
Helm - Kira's Guardian (4 uber Emeralds)
Body - Tyrael's Might (4 uber Emeralds) / Holy crafted Archon Plate (4 uber Emeralds)
Belt - Nosferatu's Coil
Hands - Soulmancer's Grasp
Feet - Soulmancer's March
Rings/Amulet - Elemental set