I managed to get a 100% completion - fun game, I enjoyed it.
The knight girl is super busted. Grab as many of the Broken cards as possible and just stack them on the enemy. Most of them give you defense as well, so your gameplay is basically just turtling + spamming Broken, and then either stunlocking the enemy with the card that stuns if the enemy has 4+ Broken, gradually building Attack with the card that gives perma-attack if the enemy is broken, or using the shield->attack when you have tons of shield. I beat the last boss just by stacking 20+ Broken and then stunlocking it every turn.
For the thief girl, literally just grab the artifact that gives +10 special skill, and stack bleed. Kill everything in 1-2 turns, lol.