Boss fight with a monster named Xel.
My usual way of dealing with bosses is using a swarm deck based around Deathwatch. It's a keyword that triggers whenever any minions on either team die, and has different effects depending on the card. The two main minions that form the basis of this deck are Shadow Watcher (gains +1 attack and +1 health when anything dies) and Shadowdancer (damages the enemy general for 1 and heals your general for 1 when anything dies).
They're effective against bosses because the AI is kind of dumb and will keep killing your minions until your Shadowdancer kills them or your Shadow Watcher gets so strong it can one-shot them.
Well, they finally added a boss that uses Deathwatch itself. Specifically, it has the Shadowdancer's ability built-in, and also damages your general every turn based on how many minions you have on the board. The boss fight also
helpfully starts you with two weak minions on YOUR side that respawn whenever they're killed, so unless you find a way to get rid of them they're a massive liability since the enemy can kill them over and over to damage you.
So I tried my usual Deathwatch swarm deck and got obliterated.
Then I tried all my other decks and got obliterated. It turns out my deck-building style always tends towards swarm tactics, which is suicide here.
I eventually made a bizarre custom deck devoted entirely to winning this fight and managed to beat it. But that's the first time I've been forced to do that, so my pride is still a bit wounded...
Duelyst.
It turns out the secret to winning was to out-Deathwatch it with MORE Deathwatch.