In the Shop tab you will have several tabs, but the one I want to focus mainly on is the Sapphire Shop tab which lets you make some purchases for Sapphires. I would suggest that you buy the Imp for 30 days for 666 Sapphires, because it is going to let you pick up boss loot automatically, and also give you +50% killing boss EXP. That and also the Cherub are good, but now it all depends on you and how you want to spend your Sapphires.