Skip the first troll:
When you reach the troll a cutscene will play showing him getting ready to face you. After it ends send all of your minions to open the gate where the crane is, and rush through it. Then go to the crane and the game will autosave. After the autosave leave the area and come back. The troll will have disappeared.
Respawning treasure chest:
In the Infernal Abyss where you meet Kahn for the second time, next to him is a narrow hallway that leads to four spawning pits for each minion and a treasure chest. The treasure chest will respawn about every 10 seconds.
Easy Minion Souls:
Go to your dungeon and get out reds, blues, and browns/greens. Select the type of beetle you wish to fight depending on which type of soul you want (red for Magma, blue or Dazzler, 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. Continue to repeat this tactic. You will kill them all with a maximum loss of one minion. However, you will gain about 50 to 75 Minion Souls.