So if you thought like me that you could one shot Folneus then you are wrong:
PSA: Max gold is 9999
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.
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.
When I played the game on Android it was indeed my favoutite strategy as well. The Death card strategy still is and having even more copies than 3 not always is that bad because it means that the card will come sooner. Right now (at least while playing the Trial Tower) I don't really take Surprise Strategy because either I kill everything one or second turn even without it or the enemy with half HP is still too tough to take it down.
Right now my strategy revolve around that triangle artifact that increases cards' attack by 1 for each 10 hits. My key cards are the one that either do multiple hits and/or attack all enemies at once (Gambler's Knife or Sheep Progress) or are multiple cards in one (Foamy Sword or Soda Water) or are Covering Fire. With this easily you can deal even 500 damage a turn. Example of a single Trial Tower run from today:
Ignore the starting cards (in later Karma sets removing them costs far too much that only key cards like useless Amulet can be removed). The Grow Up Ray card I usually take only for fun to see how much I can rise it. Otherwise it's useless as well.
Right now my strategy revolve around that triangle artifact that increases cards' attack by 1 for each 10 hits. My key cards are the one that either do multiple hits and/or attack all enemies at once (Gambler's Knife or Sheep Progress) or are multiple cards in one (Foamy Sword or Soda Water) or are Covering Fire. With this easily you can deal even 500 damage a turn. Example of a single Trial Tower run from today:
Ignore the starting cards (in later Karma sets removing them costs far too much that only key cards like useless Amulet can be removed). The Grow Up Ray card I usually take only for fun to see how much I can rise it. Otherwise it's useless as well.