create cast macro's for your slaves
eg.

Main follow macro
/target Mainname
/follow

Lightning Bolt
/cast [target=targettarget, harm] [] Lightning Bolt

Earth Shock
/cast [target=targettarget, harm] [] Earth Shock

Healing
/cast [target=targettarget, help] [] Healing Wave

etc

There are a few threads out there that show you how to box without using focus or assist.
Personally my slaves only ever stop targeting my main if they are looting or meleeing.