i just use:

/assist party1
/cast spell

this way is simple, and doesn't matter if your 5 boxing or 4 boxing with a friend tanking, still assists the right person.
it may not be as simple as changing focus when your main dies but u can promote someone else to leader when u die if u want to do it that way. but i figure if my tank dies i'm usually SOL anyway and start working on wipe recovery asap (ie: battle res the tank before the druid dies, or move the shaman to a safe location for ankh)