For a focus based assist system, you have a hotkey to switch which toon is active.

If you have a program with PIP (Picture in Picture) capability, such as Keyclone, you would have the same hotkey for both switching which toon is in the foreground (Keyclone PIP key) and for a warcraft macro to switch who your leader is. The warcraft macro would include, "/focus NewLeaderName" and possibly, "/script PromoteToLeader("NewLeaderName") ". For a focus based leader system, you'll have one hotkey per toon and every toon will click the hotkey. So if you have a 5 box set up, you have 5 macro's for leadership changes on each character. Pressing the macro for #4 will have all toon's, even toon #4, press the macro which makes #4 their focus.

Now that the correct leader is the focus, all of your spells cast on the target of your focus.

If you want to have your slave toons target the leader's target, your spells will look like this:

/assist Focus
/cast Spell Name

/assist Focus
/castsequence Spell 1, Spell 2

If you would prefer to have your slaves hit the focus's target without a target themselves, instead use these:

/cast [target=focustarget] Spell Name

/castsequence [target=focustarget] Spell 1, Spell 2, Spell 3......