In the watchtower level 3 (I believe it's level 3) almost half way through the level there is a room where 2 mines come out of the floor the next room has a save point and heading up to the next room there the pulse thing that slows you down where 4 mines are continually generated.(I was playing single player and let Batman die so he wouldn't be in the way).

Using a character that can destroy the mines in one blow (I had used Wonder Woman with her Tiara toss) Your able to gather boost gems galore in either one of the rooms. Well at least as many as you can carry.(35) ( The mines also drop Healths, Energy, and Shields)....once your fully stocked head out to the save room, Combine the gems and create more powerful ones and then head back into the room and start again.