you don't want to imbed an /assist into your cast macros because there will be times when you want your main attackign something, while your alts continue to burn down the target you started with. Now what you can do is this:

Code:
/cast [harm,nodead] Lighting bolt
/stopmacro
/cast [target=focustarget] lighting bolt
Assuming you set your main as your focus for all your alts, this macro will continue to spam lighting bolt on whatever target you initially told them to assist, and then automatically switch to your new target once it is dead.