I just have /cast Focus Magic.
It lands on a random party member.
Since no matter who I lead with, I target the mob when I push my "click" spam key.

The other toons, auto assist the hot toon.
And cast at the mob.

The mage never actually targets the party.
But after a session of grinding mobs, be it in an instance or several outdoor mobs in succession.
2-3 members of the party have the buff.

I'd assume, any one of them gets a crit, and the mage then gets the bonus from the spell.