The point of [focus] is that any toon can lead.
And all of the others follow that toons lead.

Everyone auto attacks the lead toons target.
You don't need to use assist and then cast.
Just cast.

Focus based assist costs you the Focus ability on others.
But its the easiest assist method to start with.
And lets anyone on the team lead.
It is a good starting point.

In the end, use whatever works for you.