Hearthpwn

The combo is:
2 Mechwarper (3 mana)
1 Kanrethad Ebonlocke (+0 mana)
1 Old Murk (+1=4 mana)
1 Skycap (+0 mana)
2 PO on Skycap (+2 = 6 mana)
1 Snip on Skycap (+0 mana)
4 Snip on Old Murk (+4 = 10 mana)
Attack with Skycap for 14 dmg (4+2*4+2)
Sac pact Skycap to fill board
Attack with Old Murk for 16 dmg (2+6*1+4*2)
30 dmg OTK!
The most flexible part of that combo are the 4 Snip-Snaps on Old Murk. You can discount 1 Snip and 1 mana (up to 4, but actually up to 10 but more complicated) for every...
- If you soulfire last
- 2 enemy minons (b/c Old Murk)
- 2 enemy hero health missing
- twice that if you discounted a mechwarper or Kanrethad with Free Admission; tripled if both.

You can also count missing conditions, but with 3 or more conditions no matter if positive or negative you're probably better off calculating from scratch.
Example: You desperately tap + Manafeeder into Free Admission which gives you a Mechwarper and your Skycap. You now have all combo pieces except Sac Pact. Also the enemy has 5 minions on board. You only have 3 mana left, but that's actually still 20 dmg with incomplete combo!

Note that if for example on T5 a Crabrider sticked on board, then you can and 90% totally should 2x PO +1 Snip for 5 mana and 22 dmg.

Mulligan: Don't keep any of the finishers. Keep the crabs, Kobold Librarians and Free Admissions. If you're sure, that you're going to get swarmed early on, then keep Defiles, though they are usually stronger in mid-game, than early on.
Against Pala keep Mortal Coil.
Against Pala and Shaman keep Crab Rider if you can also keep a PO.

It's okay to play vanilla manafeeder if you have handsize issues. Vanilla crab rider is preferable to manafeeder.

The deck is expensive. The version with 2 Expired Merchants to generate 2 Old Murks is a tiny bit cheaper and works just as fine.