hero merit grinding

Grind tempest trials.

If you haven't completed one of the 10-map Chain Challenges on Normal yet, you can grind HM (and SP!) pretty easily there. Play through maps 1 to 9, then forfeit on the final map. That way you don't get the completion penalty and you can play it as much as you want.

1. Choose 2 or 3 tanks, or a tank & a sweeper.
2. Put them in a team with a bulky healer (Azama) or a dancer, or both if you chose 2 tanks.
3. Do Tempest trials that are 5 or 10 levels lower than the level of your team. For example, I run a lv 40 tank team on the lv 35 lunatic map set.
4. Repeat and earn rewards + merit!