If you're making a lightning tree sorceress, I would actually suggest equipping your sorceress with infinity instead of your mercenary. Making infinity out of a 4os Voulge has 2 distinct advantages:
(thanks to GotFriana for the infinity stats)
Level 12 Conviction AND -(45-55)% To Enemy Light Res (only applies to the character wearing it):
that works out to somewhere between -130% to -140% to enemy lightning resistances. It breaks just about every lightning immunity.
It also reduces enemy defence by a hefty percentage.
0.5-49.5 To Vitality (Based on Character Level):
nice life bonus!
Level 21 Cyclone Armor (30 Charges):
for additional elemental protection
The only disadvantage is that you don't have a shield to add to your defense rating. However, your killing power is greatly increased, and you won't be exposed long enough (usually) to be in serious danger.
For your mercenary, I would suggest a few options:
Act 2 Holy Freeze (crowd control)
-Pride runeword (Concentration) for damage boost
-Reaper's Toll (Decrepify) - helps with crowd control
Act 2 Defiance (boosts defense)
-Pride Runeword to boost damage
-Doom (Holy Freeze)
Act 2 Might (pure damage)
-Pride for another damage boost
-Holy Freeze for crowd control