Marvel Puzzle Quest: Dark Reign

Thick as Thieves is the 3rd chapter in the game. (The first is just a tutorial)
There are 4 missions within this chapter as well.

1: Family Reunion has 8 battles and enemies range from level 115-130
2: Anderson inc. has 7 battles and enemies range from level 130-145
3: Scotland Yard has 9 battles and enemies range from level 145-170
4: Thick as Thieves has 11 battles and enemies range from level 160-180

So there are a total of 35 battles you must play and you will want to have your best fighters around the level 200 range by the end of this chapter. If you are having trouble, replay earlier missions to gain levels and replay the first fight in the game repeatedly for easy wins to earn more covers to boost your powers.