Ya seems to be a recent update that changed that fight (as well as a bunch of other stuff, including the hermit). Used to be 1 at a time, like you said. I played twice since this change, and beat it both times (but at low ascencion levels, 1 and 0). One was with the slime boss, the other with the snecko.
Slime boss was a minion build. I relied on summoning slimes and buffing them with the power that gives +4 effectiveness to slimes but goes away in a few turns. I had two cards that duplicate, and I tried to duplicate that power. I also had a power that makes me command more, and a skill that commands for x. So I tried to win very quickly, before the power ran out and my slimes became weak again. That elite fight was pretty easy for my build because I made use of the aoe slime to quickly kill them all.

My snecko build was close to an infinite build, it played like one. It made use of cheap non-class cards, and that one power that draws a card when I play a non-class card. I had 4 of the power that reduces a non-class cards cost by 1 at the start of your turn. I had rampage, that ironclad card that gets more powerful every time it's played. I had non-class cards that generate 0 cost defense cards, and these would be my draw engine and sustain. I would be able to draw through the deck multiple times a turn and cast rampage multiple times. Some fights I think I did reach infinite. The elite fight wasn't.

Good luck on you future runs!

Thsnks. I thought maybe i got a different mod, but yeah noticed recent changes like the hexaghost card "Speed run", used to be "light a fire, move to next fire", now it's "when you advance to next fire, get 2 temp str".

No prob. Didn't know they changed the hexaghost, too. Haven't played that class in a while, I'll have to give it another go to try out the new changes.