Thought you might find this interesting?
https://www.youtube.com/watch?v=-hzdTUi4LL8Meteor + CB, meteor stops health regen it appears, and CB when done point-blank, hits all bolts dealing a pretty consistent 20-25k damage.
Conviction wont cut mephisto because his conviction level will be higher than your mercs, but get a low res wand and it'll help.
If I had to guess what this skill setup and gear looks like, I would say gear is pretty much what ever you can muster for survival and damage output.
Build:
20 pts chargedbolt
20 pts lightning
20 pts lightning mastery
1 pt static
1 pt teleport
1 pt warmth
3 pts pre-requisites
20 pts meteor
20 pts fire mastery (7% bonus versus 5% from skills synergy)
rest fire ball