Maybe we're going about it the wrong way. Instead of trying to fix it as a cantrip, maybe rename it to "Focused Strike" or whatever and make it a normal combat action that is available to everyone, like Dodge or Disengage:
Focused Strike
In combat, you may use your Action to focus on a single target within range that you can see. If you do, your next attack roll against that target is made with Advantage.
Again, not perfect. But I think it's better than making it a cantrip that nobody will ever choose.