You can add multiple targets to the macro, in the event that one doesn't exist.

/assist party1
/cast [target=party1target,exists,harm,mod:shift][target=target,mod:shift][mod:shift] Frostbolt
/cast [target=party1target,exists,harm][target=target,mod:shift][] Fireball

crude, but that way if your tank dies the mage will still nuke ther last target, or whoever they end up targeting. instead of [target=target] you could put [target=party3target] or any other party member. As long as you dont run into the macro limit, you could put a conditional progression of targeting leaders to fall back to in the event of a toon death. It's not flexible like the "TFL" setups, but still pretty powerful. If your tank dies, just pick up the next toon and fire away, and the rest of the toons should see that the tank has no target, and move to the next toon for targeting, which you are controlling. Since I still use a focus system, I think I will try to adapt this system for me, but all I need to do is make one focusing macro fallback, so the toons focus on someone else in the event of main death.