Why not let the players handle combat in whatever way they come up with? Focus fire doesn't seem like a problem, it just seems like the players not doing what this DM wants. Trying to control player behaviour looks like a DM problem, not a player problem.
That said, one thing we don't like at my table is players meta gaming and telling each other what to do on their turns. So if the focus fire is happening because of that, then that would be a problem. But if it is just players being strategic...it's their game.