The problem with that macro is that any tank worth his salt is going to be switching targets a lot. You don't want to follow the tank's targeting around. You want to focus on the first kill target until that's dead, then move on to the next.

I'd suggest setting the tank as your focus (I don't remember the default keybind for this. Ctrl+F, perhaps. The macro command is /focustarget), and then using a macro that's just "/assist <f>" to acquire the tank's target once per enemy.

I'm not sure how smart or stupid /assist is. If it's smart enough to know you can't really assist hostiles, you could use:
/assist <t>
/ac "Something" <t>

But if /assist is stupid, that'll just end up swapping targets back and forth between tank and mob.