Found a totally broken combo, got to level 54 before getting bored and letting myself die. Here it is:
freeze + next shots bounce + 9 bullet shot + powerup: bouncing bullets pierce
9 piercing bullets that also bounce back to piece multiple times just completely shreds entire screens of enemies, freeze lets you cycle through your cards for free and gives you loads of time to find the right card, and the "next shots bounce" card is also free so you can have many of them and just burn them until you get to your 9 shot card.
Feel like it's a bug/oversight that "next card bullets bounce" is a free card, since the "next card bullets pierce" is a 1 cost card.
My combo would probably still work since the freezes give you time to regen your energy, but occasionally the combo would become weak because of the forced card swaps and it might be enough to get unlucky and die