Answered I don't have a Prayer

Nechyrael's are faster in terms of infernal ashes/hr, that is if you only use the regular method at black demons.

Technically speaking, you can lure a black demon on top of you while using a cannon, standing ontop of the cannon this demon will demon stuck and your cannon is free to attack other enemies. You can easily reload your cannon at this point without trouble. Doing this is technically 500 kills per hour. The problem here being you only receive ranged experience and it can get quite costly.

While there are other methods these two are the standouts of them all.