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.
Respawning treasure chest
When you meet Kahn for a second time in the Infernal Abyss, he will be plastered to a metal wall in a small room. Next to him is a small hallway that leads to four spawning pits for each Minion and one treasure chest. The treasure chest will respawn every eleven seconds after you hit or open it. Note: The Infernal Abyss is the fifth and last one.
Skipping the First Troll
On Legendary, the first troll is very hard and consumes alot of minions. There's an easy way to completely skip him (Make sure you have 10 minions):
Head straight to the troll, and you'll get the cutscene of him getting ready to face you. After that, send all of your minions to open the gate where the crane is, and rush there (the troll will probably kill a few of your minions as they open the gate, but that's better than wasting 20 - 40 minions trying to kill him and also, if the troll is blocking the path, simple lead him away and head through the gate).
Once done, go to the crane and an autosave will occur. This is where the trick comes in: That autosave assumes you defeated the troll as you went further than you were suppose to, therefore, simply leave the area and come back, you'll find the troll has vanished as the game thinks he has been defeated.