Might merc, Faith weapon, and Mephisto's Claw with two Chiad's Halo rings for (4 to 6) +2*4 = level 12 to 14 Holy Freeze aura. Also use Lysiann's Eyecatcher somewhere (character if your damage is important and not just your minions') for the level 20 to 25 Concentration aura.
I don't like to have my attack speed dependent on my merc, so I generally prefer either an Infinity polearm (level 12 Conviction) or Pride polearm (level 17-19 Concentration).
If you use and Act 5 merc, you can put a Prowess in Battle runeword barb helm on him for a level 6 Fanaticism Aura. With Faith that gives level 18 to 21 Fanaticism.
You could use Lysiann's Eyecatcher+Pride Polearm+Mother's Milk (unique armor, level 1 to 6 Concentration when equipped) for level 39 to 50 Concentration. I'd skip the Mother's Milk and use an armor that would better protect my merc.
Maybe this will help:
Aura,Level of Aura,Name,Quality,Type of Equip,Level Req
When Equipped
might,17,Last Wish,runeword,Swords/Axes/Hammers/Druid Hammers,65
concentration,6,Mystery,runeword,Druid Pelts,69
concentration,17 to 19,Pride,runeword,Polearm,67
fanaticism,9,Beast,runeword,Axes/Scepters/Hammers/Druid Hammers,63
fanaticism,12 to 15,Faith,runeword,Bows & Crossbows/Javelins/Throwing,65
fanaticism,6,Prowess in Battle,runeword,Barbarian Helms,70
conviction,4,Daylight,runeword,sorceress orb,65
conviction,12,Infinity,runeword,Polearm,63
might,1,Gemini Coat,unique,armor,12
might,1,Ring of Engagement,unique,ring,3
concentration,1 to 6,Mother's Milk,unique,armor,51
concentration,20 to 25,Lysiann's Eyecatcher,unique,boots,66
fanaticism,1,Razor Strike,unique,bow,55
conviction,5,Athena's Tirade,unique,sorceress orb,78
conviction,1,Star of Bethlehem,unique,sorceress orb,59
oskills
fanaticism ,3,Terror,runeword,Necromancer Shrunken Heads,49
concentration,5,Blood Raven's Dispair (helm/armor/bow),whole set,set bonus,77
concentration,1 to 2,Knight's Dawn,unique,shield,29
conviction,10 to 20,The Band of Chaos Empire,super unique,belt,81
Aura/Per level benefit to party after first level:
Might/10% damage
Concentration/12% damage
Fanaticism/7.5% damage, IAS increase (varies)
Conviction/-2% enemy resistances, -% enemy defense (varies)