Code:
/cast [Target=FocusTarget] SpellNameHere()
and have a key for all that does something like for all the other boxes

Code:
/focus [nomod] PaladinsName
/focus [mod] BackupTankOrEmergancyPlayer
As long as the paladin is the focus, they'll use his target to cast their spell on.
If he dies, you hit a mod (Ctrl, Alt, or Shift) and the focus button, which will re-focus to the character specified - in which case all the players will now use this focus as their "assist".