Diablo II Act 2 Mercenary Auras

Combat (Normal): Prayer
Combat (Nightmare): Thorns
Combat (Hell): Prayer

Defense (Normal): Defiance
Defense (Nightmare): Holy Freeze
Defense (Hell): Defiance

Offense (Normal): Blessed Aim
Offense (Nightmare): Might
Offense (Hell): Blessed Aim

Tips about Act 2 Hirings:

Act 2 hiring with Prayer Aura and Insight Runeword:

For Act 2 hirings 75 level and up, their Prayer Aura is level 18, which heals 21 life very 2 seconds. When equipped with Insight runeword, it heals 42 life very 2 seconds. It is because of the synergy between Prayer Aura and Mediation Aura. Synergy is only in effect when hard skill points are spent. Mercenary native auras are taken as hard skill points in this case. 

If equipped with +2 All Skill armor (for example, ebugged Chains of Honor) and +2 All Skill helm (for example, Andariel's Visage), with +1 All Skill from Battle Orders (Call to Arms or Barbarian Skill), Prayer Aura is 23 level, healing 31 life every 2 seconds. The number doubles when equipped with Insight runeword. It heals 62 life every 2 seconds.

Do Prayer Aura from different players/mercenaries stack?

 It depends. Only the highest level aura is in effect. If the Prayer Aura and Meditation Aura of Insight Runeword are exactly the same. They stack. For example, if all 8 players in a game have hirings with 23 level Prayer Aura and 15 level Meditation Aura (from Insight Runeword), 496 life is healed every 2 seconds.

Are Prayer mercenaries hired from Normal difficulty and Hell Difficulty the same?

No. Combat mercenaires hired from Normal difficulty has level 18 Prayer Aura if they are 75 level or higher. Combat mercenaries hired from Hell difficulty has only level 16 Prayer Aura if they are 98 level or higher.