Imp phased out with only Blood Pact on. He regenerates mana the fastest, and he's invisible so he won't get attacked.
You can lead with:
Shadow Bolt or Soul Fire
then DoTs
then life tap followed by drain life. Important to have Corruption and Drain Life going to get Shadowtrance from Nightfall. Throw in the instant SBolt whenever you get it. I also have improved drain soul, so I get 15% mana return + soul shard if I use it and kill the enemy. Any mana you are missing after this, take from the imp.