Something like VT > DP > Mind Blast with orbs > Mind Flay. SW:D under 25% and to gain mana back, SW:P should be refreshed through Mind Flay. Keep Empowered Shadow and Dark Evangelism up. Use Dark Archangel when you'll have time to Mind Flay twice before recasting VT or DP. Use Shadowfiend on cooldown.
I've been finding two things, depending on what you're killing and if a mana break afterwards is possible:
The long fight; mobs with more than 200k health, or a flight against many mobs/boss:
SW:P, DP, VT, MF (repeat as required until you get 3 orbs), Dark Archangel, MB (with the 3 orbs), refresh VT and DP if required, repeat at MF.
The short fight; mobs under 200k health with a mana break possible:
MS x3, MB, then either SW:D twice (if mob has ~30k HP left and that's under 25%), else repeat. Either target will die halfway through the second run, or the SW:D of the repeat will kill it.
I'm unsure if MS with two stacks is "better" than MS with three, in the sense that casting MB with two stacks makes the MB insta-cast and a crit-chance of 60%, where as, if I recall correctly, 3 stacks makes it a 100% crit chance. I guess that's one for the theory crafters?