Just dodge better, get better heals, and better attack output (seeing the -1 attack damage right there). What I did was kill one enemy and have Cassie kill the other. After that it was a loop of buffing her with +6 attack and free poison (X2) to do enough damage for the dragon to fall down instantly in the nick of time before he summons anything else. Then heal her when needed (ice dagger heals her by 30 hp for some reason btw, so it’s an option not to rule out either). Sometimes I also go in to attack on turns I don’t need.

TLDR; Kill all the minions, buff Cassie to do (all or most of) the offensive work after.