After beating the game, u will have to attempt solitare's challenge, well after u beat the challenge and Solitare herself, you will be given a brass key. After u get it, go to the sewer and find a metal door. Use the brass key to open it and you will find another room, which happens to be Snap's reserch lab, and in the back is the Estark. If you beat it, it will join your team. It should be at lv30.