The thing about felguards is they put out pretty good DPS by themselves, they are like a poweful dot that you only need to cast once per target, without the need to constantly refresh it every 20 to 30 seconds. That means if things get a little hectic, and you are having trouble remembering to cast, or if one or more of your warlocks are getting cc'd or something, your felguards are still doing a ton of dmg. This makes them an excellent choice for a multiboxer.

I would possibly consider swapping one out for an imp for the stamina bonus, but you would probably take a noticable dip in DPS.

Also, you can round robin the charge on your felguards, so as you are kiling mobs individually, each one is stunned where it's standing, then dies a few seconds later. Felguards are so awesome.

Plus they have cleave. 3 or more felguards all cleaving can make pretty short work of a small group of mobs, even without the help of the warlocks themselves.

If it were me, I would COA or Corruption (COA is much cheaper mana wise, but takes a bit longer to ramp up in DMG) while sending my felguard in to attack simutanously, then just ran past that mob and repeat on the next mob. You should hardly even have to stop moving, they will just die behind you, the dots finishing them off after the felguards softened them up a bit. With Mana tap you could probably use corruption instead of CAO and get a bit more DPS right off the bat. Maybe immolation would be good too. Make sure you train the ability that makes your felguard get mana everytime you lifetap, otherwise all that cleaving and charging will leave them manaless in no time.