I've recently switched over to use focus for following and assisting.
This causes a slight problem when I want to sheep/shackle/banish a secondary target repeatedly while fighting another mob.

I've come up with a solution for my group, but I'm wondering if anyone has any suggestions to make it more efficient.
Right now, I don't have a focus set on the party leader, so I use that for the target I want to keep out of the picture.
I just have to click the focus..and then hit my macro key.

Here's what I have so far (I'm doing this from memory, so the set raid target isn't correct).

Paladin keys:

Alt1 key:
/stopattack
#/set raid target 7
/focus

Alt2 key:
/stopattack
#/set raid target 8
/focus

Alt3 key:
/stopattack
#/set raid target 5
/focus

Mage Key:

Alt1 key:
/stopcasting
/assist [target=focus, noharm]
/assist [target=focus, noharm]
/p Sheeping %T
/cast Polymorph

Priest Key:

Alt2 key:
/stopcasting
/assist [target=focus, noharm]
/assist [target=focus, noharm]
/p Shackling %T
/cast Shackle Undead

WarlockKey:

Alt3 key:
/stopcasting
/assist [target=focus, noharm]
/assist [target=focus, noharm]
/p Banishing %T
/cast Banish