Who a monster goes for

A monster fights one frog at a time. Which one is not luck: it follows a few plain rules, the same for every monster, and you can play with them.

A monster that sees you, or is struck by you, comes for you.
Every blow on a monster is threat: the monster remembers who hurt it, and how much. That memory fades over a few seconds when the hurting stops.
Healing is threat too. Mend a friend a monster is fighting and it notices you, a little (a quarter of what you healed). A healer who stands back is safer, never untouchable.
A Druid's wolves earn their own threat with every bite (twice as much as a blow), so a monster often turns on the wolves instead of the Druid. A heal on the wolves is the Druid's threat.

A monster keeps its target until someone else has earned clearly more threat (half again as much), or its target is gone, dead or out of sight.
It never flips back and forth: after it turns, it stays on the new frog for at least a second and a half.
Now and then (about one turn in five, every four seconds) a monster in a long fight turns on someone else in it, weighted by threat. Bosses never do this.

Knights taunt. Challenge (one monster, 5 tiles) and Taunt Roar (all around him) make monsters come for the Knight for 4 seconds, whatever anyone did. A Knight who keeps taunting holds a monster nearly the whole fight, even against a Mage who hits harder. Bosses listen only briefly, then ignore taunts for a while.
Rogues pull. Twin Strike, thrown from up to 5 tiles, turns the struck monster on the Rogue for 3 seconds, once every 12 seconds per monster. Thrown, the blades cut at half strength: it is a pull, not a way to fight from afar. A Knight's taunt wins over a pull, and a boss only grows angrier at the Rogue.

Pull one monster to your group, not a crowd onto a frog who is not fighting.
Let the Knight take the monster off the healer; the healer keeps the Knight standing.