I just use spells normally without macros, but have /assist main on my thumb button on my mouse and i spam that every time i change target.

My 3 slaves also have 'set focus' on F4, F8 and F12 , and using trinity bars i can Ctrl + spell to cast on the focus.

So for CC i can set 3 different focus targets on my 2 warlocks & mage, have lock1 focus fear on Ctrl-R, lock2 focus fear, on Ctrl-T and mage focus sheep on Ctrl B whilst hitting my spells keybinds without Ctrl to attack my main's target. (i wrote macros for these so they /target lasttarget after firing off the cc)

I also have keybinds on Insert and Page up to /assist pet1 & /assist pet2, so for PVE where i have time to 'assign' targets before i commence a fight I can effectively switch between up to 7 targets easily - focus1, focus2, focus3, focus main, pet1 target, pet2 target and main target.

Seems to be working well so far for Pve, but I am not sure how easy it will be to use this system in the midst of a hectic pvp battle.