Since I got asked, here’s a simple Crowd Control macro which will set the controlled mob to your focus. We’ll use the mage’s sheep for that:
/cast [target=focus, exists, nodead, harm][harm] Polymorph
This will try to sheep your current focus provided it’s alive and an enemy. If not, it will sheep your current target if it’s an enemy. Once done, it will set it as your focus.
If you hold down the alt key, it will clear your current focus instead, and do nothing more. If you want instead to clear the current focus and then poly your current target, remove the line with /stopmacro [modifier:alt].
Ideally, you’ll use some unitframes which does show your focus (don’t even know whether the default UI does that).