You are right, firebats aren't casted while killing Elites. Mostly because firebats have a buildt in mob detector i think so it wont waste mana on one mob.
A very good build ive found to work perfectly for all is
summon zombie dogs (life link)
corpse spiders (widowmakers)
Spirit walk (healing journey)
Locust swarm (pestilience)
Firebats (Cloud of bats) I also recommend using plague bats on this. It cast it perfectly.
Soul harvest (siphon)
I used plague bats on earlier versions of trinity and it works perfectly against any mobs.