Seems to me like this is well worth it. On black demon tasks I was getting at least 200 kills per hour along with 330k fletch, and on dark beasts it was 185 with, again, 330k fletch.

 

You just fletch 10 sets of broads, refill cannon, and repeat. You do miss the melee xp though, which you might care about.

 

 

I was doing it with 256 range attack, and getting 25xp per cannon ball on the black demons, and 20.5 on the dark beasts.

 

(you wouldn't have to be on a slayer task to make use of this, of course, because the effigies might make it worth it by themselves, but I personally wouldn't bother with it if you're not).

I might try this next time I cannon a task. I'm maxed melee so I don't need that.

 

But I don't get where people cannon black demons? Tav doesn't seem worth it to me.

 

Where do you cannon dark beasts?

For dark beasts it's useful to have done within the light so you can tele to the light temple with an elf crystal. You cannon them in the northeast room, where you are able to step into a narrow tunnel to allow the cannon to multi-target.

 

Black demons you do in the tav dungeon and hide behind some rocks. In both cases higher population worlda are best.