Okay, I guess I'm the slow one on this quest... I read all the comments and tried to do this in-game and took FOREVER standing-around trying to figure out what I was supposed to do, and I couldn't figure-out how to get my cannon to fire once I took-it under control. I kept reading comments above about imps and I was saying to myself "What Imps? I don't see any imps!".. so if anyone else is a bit slow like me, here's what you do:
1) take-over the cannon (if you can't figure this out, you're slower then me!)
2) you'll get a "pet action bar" over your own bar (I somehow missed this)... don't summon any pets after taking-over the cannon or you'll lose the cannon!
3) don't use "attack"... for those who use pets, that's not what you use! I sat around for seemingly 10 minutes trying to get my cannon to "attack" once I realized there was a pet-action bar!
4) there's two seperate attack modes on the pet action bar that you do use to attack... kinda colored green and blobish looking... use those to start the attack!
5) once you get-off the first shot, imps will start coming out of the teleporter to attack (suicide bomb) your fel cannon... kill them as quick as you can by any means necessary... they don't have much health but come fast... any area-effect weapons should do quite well here... I used my Death and Decay AoE as a Death Knight... worked quite-well at wiping-out multiple imps.
6) wait for the "cooldown" of the fel cannon and hit it again (and again, and again)... it will take multiple shots to destroy the portal, all while imps try to kamikaze into your fel cannon... kill the imps at all costs.
That's pretty much it... I had a hard time figuring out what I was supposed to do for this quest, so if you're like me, I hope the above helps out!