If your slave has you set as the focus, I think what you want instead is something like:

Code:
/assist [target=focus]
/cast Sinister Strike
/targetlasttarget
Macros do not support variables (scripts do, however), so you can't do anything like "#MOB#". If you know the exact name of the mob, you can type it into the macro, but then the macro would only be good for that exact mob, and you'd need a ton of macros. Handy for very special encounters, though.