I started marking (good) arcanists with one of the tab target icons so my pug teammates focus them down faster. Both Ceph Flail and Fatecarver are a lot worse when they're surrounded. Turned a solo queue match around just by focusing their arcanist—who originally seemed unbeatable
Doesn't fix the balance issue, of course, because they're still strong. Just my experience this weekend in mid-MMR BGs