Go to your dungeon and get out blues, reds, and browns/greens (your choice). Select one of the types of beetles that you want to fight. Depending on which soul you need, you must pick the beetle for that (blue or Dazzler, red for Magma, brown for Battler, or green for Puff). Enter the arena. Take all the minions and place a marker. Move it all the way back up against the gate. Go over to the beetles, but only to get one, two, or three. If you get too many, lock on to one then back away quickly. The extra beetle will start to fall back, then stop. Let the others catch up, but not touch you. Run backwards until you reach your minions. The reds will already have its health down a small fraction, then the brown or green will start attacking. You will probably have one or two minions die, so the blue will move in and heal them. Keep repeating that tactic. You will kill them all with a maximum loss of one minion. However, you will gain about 50 to 75 Minion Souls.