Welcome to Librarium Online!
I watched my friend (Chaos player) and my other friend (Bret) go at it in a 2k point game.
The hellcannon wreaked absolute havoc on the brets, and not through shooting. It just ate them in close combat, which was ridiculous!
How do ya'll fight them, im having a hard time thinking of a way to kill it/render it useless. I can take out the dwarves with Carrion, but then it just runs at me and smashes my lines.
Make sure you read the rules for the hellcannon pretty closely, they are reasonably complex compared to most things.
It is a very devastating piece of equipment, but it also has the potential to ruin your opponent's units. I don't remember exactly off the top of my head, but I'm pretty sure if you kill the Dwarfs (as you suggested) it will just go nuts at the closest thing it can reach, friend or foe. So take them out early when it is only close to enemy units. You can also use the frenzy to redirect it away from the battle with things like carrion, but that is less effective for undead because you can't elect to flee from the charge.