Go to your dungeon and get out reds, blues, and browns/greens. Pick 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, then 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 begin to attack. You will probably have one or two minions die, so the blue will move in and heal them. Repeat this tactic to kill them all with a maximum loss of one minion. However, you will gain about 50 to 75 Minion Souls.
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.
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 aroundevery 10 seconds.
When forging, use various types of minions for better results. Using only one type of minion will only boost one attribute. For example, when forging a steel sword, if you put 100 brown minions to upgrade you will only increase damage. However, if you use 50 brown and 50 red you can increase damage and fire damage.
A great area to get lives for brown minions is the first area(take left road). If you go after unlocking the first forge, you can find about 40 sheepies (in other words 40 lives) just kill the sheepies (they are extremely easy) and you can get the lives.