This is more just general tips as the combat in this game is somewhat open ended as far as strategy goes.

1 - Do not change characters mid run. This of course saves the time for backing out and coming back in.

2 - Don't use the Seeking Shrunken spell against opponents near you. It has a long startup and finish, so that or other opponents can easily hit you in that time.

3 - Have lightning or fire on deck at all times for quick retaliation

4 - In boar polo map 1, ride the boar, hit the ball against the central wall on the opponents side, then, knock the ball towards the goal against the second wall, running against the wall to knock the ball over. This is the quickest way to score I have found in these missions.

5 - In 3 v 1 missions, use hit and run strategies. Don't be afraid to swim out to the sharks in order to respawn where opponents are not.