I am just guessing here, but as I have the same problem I will try to flesh out what the OP is asking for. To use firebats cloud of bats skill effectively you have to hold down the key it is assigned to such as the RMB. If you let it go, and re click it costs a huge amount of mana as opposed to just holding down the RMB and "channeling" the skill.
The popular farming spec for high end WD is:
1. Firebats - Cloud of bats - stationary channel
2. Soul harvest - vengeful spirit
3. Spirit walk - honored guest
4. Mass confusion - paranoia
5. Big bad voodoo - slam dance
6. Horror - ruthless terror or frightening aspect
The passives are:
Gruesome feast
Spiritual attunement
grave injustice
Hope this helps. If you look up lastsoulx he is basically using this same build in his mp10 videos.
Thanks,