Mulligans
General Mulligans
Eternium Rover and Armorsmith are good choices to keep in mulligan simply because they are decent 1 and 2 drops which also gain you much needed armor to stay alive long enough to play the combo. However, Armorsmith can also be useful later as a combo with cards like Blood Razor or Warpath.
Slam is much needed control and card draw to help you reach the combo.
Stonehill Defender will give you the taunt you need to keep your health up and limit your opponents trading, and can probably be exchanged for Tar Creeper.
Blood Razor is a great early weapon to clear the board. It also combos very well with Armorsmith.
Combo Mulligans
Whether or not you actually keep any of the combo pieces in your mulligan is unknown to me. It will require testing when Boomsday is released.
The idea behind this theorycraft is that by playing Explore Un’goro, then Skulking Geist, you can empty out your deck to play Mecha’thun earlier. However, there is only one easy way I can think of to kill it the turn it is played, and that would be if you were going second you could play coin into shield slam. Other than that you are praying it doesn’t get silenced, polymorphed, hexed, etc. Could be a fun deck to try if you can make it work.

























