Monster Train or Deckbuilder: the ‘good parts’ version

I finally beat a game on Covenant 1, after about 6 failures in a row. I used the Stygian Champion (even though I disparaged it earlier!), with a Hellhorned ally. I chose the champ’s -1 (eventually -2) spell cost trait, and I used that coupled with 3x Titanstooth (with emberstone reducing cost to 2 ember, so zero cost when used with the champ) as my main damage engine – 20 Frostbite/5 damage to all enemies in a row, often two-three times in one turn.

I kept the deck thin and used a couple of Offering Tokens (draw 1/discard 1 for zero cost) to cycle through my deck fast. A Guardian’s Amulet with stackstone/emberstone added 6 Sap too. The deck was so thin, in fact, that I didn’t have enough heroes for all three levels. I often just left the middle one empty and let Frostbite finish enemies on its own.

I made some mistakes. Toward the end I picked up the Imp Scholar to let me replay a consumed card. I’m not sure that was a good decision. I also stupidly spent my last gold on a needless armor upgrade for that Scholar, before I realized that I could spend that money on an artifact shop at the same er train station. Also, sometimes I couldn’t work out whether it was better to play 20 Frostbite or 6 Sap. You’d think the math would be easy, but I dunno.

So what should I do next? I just unlocked the Umbra, so I’m eager to try them. Should I do so on Covenant 1 or 2? Should I always be playing on the highest possible Covenant level?