Well, I am also just getting started multi-boxing (Atleast in the real sense, been using multiple accounts for awhile but never used key press mimic software before). A few macros ive come up with using focus for my lil priest squadron are as follows.

F5-F8 are bound to the macros that set the /focus of all my characters to a particular one, e.g F5 sets all their focus to my main, F6 to the 2nd one etc. Then I use a macro for every offensive spell as such.

/cast [target=focustarget] Smite; Smite

This allows me to not bother with the /assist command at all since they will always fire at what ever target my main one is targeted on, this also allows me to switch mains (in the unlikely event I run into something that doestn die before getting to me and it manages to kill my main) and press a different focus button to have them all fire on the 2nd ones target instead.