Basic Freeze mage deck with some tweaks to make it work on Noth and Heigan.
For Noth you want as little minions out as possible, so try to stall without using many minions, only throwing out your card generators if you have solutions in your hand to deal with the minions you'll spawn by playing them or Iceblock/Ice barrier to eat the damage you will incur. Those 5/5 HURT, so be smart about spawning them.
IMPORTANT NOTE: Noth has the Counterspell secret in his deck, so be careful you play around it and not waste your Frost Nova and watch it fizzle and ruin what was a good plan. He also has the Ice Block secret, so keep that in mind when you are getting ready for the kill.
Heigan is easier then Noth with this deck. Just play the deck like you normally would by stalling and trying to get Alexstraza + some burn spells in your hand. His Hero Power that damages your minions is of little consequence for us, since our minions are basically only there for the card draw, and whatever damage the manage to do is a bonus.
Play tips:On both bosses you try to stall and get card draw so you get your combo of Alexstraza + Burn cards (usually Frostbolt + Icelance + Icelance, or a Fireball mixed in). Stall early by using hero power and getting secrets up. Try to not use Frost Nova or Doomsayer unless you can play both in one turn. Use your health as a resource, taking damage is FINE, as long as you don't die. Don't be afraid to get hit in the face for 2 turns and watch his board get massive, if you have Frost Nova + Doomsayer in hand to wipe his board effieciently.
I tried keeping Doomsayer+Forst Nova and for the rest mulligan for Alex, since she starts your combo. The rest of the deck should be usuable as it comes up in card draws and not be needed right away.
I tried to keep the deck as low in dust cost as possible while still making it work. Some cards you can switch is an Acolyte of pain for a card like a second Flamestrike or Archmage Antonidas or another card that will help you set up a kill better and you can switch the Kobold for Bloodmage Thalnos for more card draw while keeping the spelldamage.