It's nothing special, but since I've reached Legend in 3 days I wanted to share it with you.
Quick mulligan guide:
Always keep: Backstab and Pharaoh Cat and sometimes Praise Galakrond!
With coin: EVIL Miscreant, Edwin VanCleef, SI:7 Agent
It's also good to keep Seal Fate with and without coin, but it really depends on your other cards and your opponent
It'd be easier if you have coin, but don't lose hope if you don't and if you end up having bad mulligan, you can still turn up the game very quickly.