Great job trying that out.
I haven't reached that point yet, but now that I've unlocked and used every theme deck dozens of times, I can think of one method to defeat (probably) every foe in the game in one turn, although - of course - it's hard to pull off (and yeah, one turn, not one card, unless you go for some ridiculous instant win condition - you gotta be realistic).
You need theme deck 5, 1x "Death" and 4x "Surprise Strategy" (always halves the enemy's current HP until the end of the turn, no matter how often you use the card during one turn). Good luck drawing them at the same time. Using 4x "Surprise Strategy" on an enemy with 1600 HP means he's left with 100 HP (0x: 1600, 1x: 800, 2x: 400, 3x: 200, 4x: 100), and "Death", which also works on bosses, instantly kills an enemy with lower HP than the user. Actually, since your party members in the screenshot above have surpassed the 200 HP threshold, you only need "3x Surprise Strategy" and "1x Death", which is a bit easier to achieve, especially in a deck with a greatly reduced number of cards. On the other hand, if it takes too long to gather the necessary cards, it becomes harder to pull off. In the screenshot above, one attack on your party members means the difference between having to use 4x "Surprise Strategy" instead of "3x Surprise Strategy".
It's a great strategy in general, by the way. You "only" need 3x "Death" (there's a maximum of 3 enemies per battle, and "Death" is removed upon use for the rest of the battle) and 3x or 4x "Surprise Strategy", but it's hard to obtain multiple copies of them even during longer dungeon runs. If you manage to obtain them, though, it's worth the effort, and you can imagine how well "Death" works on enemies on special floors (Clinker, Gold) that only take 1 HP damage per hit, yet feature very low HP.