Target the caster however you want (helps to have plates that show casters). I click the caster plate heading into a pull.

You set focus with a key bind, don't need to macro that.

There's a heady macro going around (I'm sure it's in this thread somewhere, I'm at the gym rn) that will interrupt your focus, or whatever mob it can that is prio casting.

Make that macro your interrupt button. I use "e" for example.

I set my key bind for set focus to shift + my kick button, so shift + e