Once you have unlocked the Summon power it can be selected with
. The first power up you get, imp will work fine. Simply call in imps and let them go kill demons. This can be upgraded so that you can call in four imps at once. There are also two other elements you can unlock by doing the alter missions. If you complete the correct alter, you will earn either tower or titan. I used a combination of imp and tower for this and it came naturally while completing the game.