"Legal" opponent OP

[Rusty]

This MF locked his "Jackpot" card right at the start, so now has over 80 Jacks! WTF. Every single turn he gets a 20, stands, and adds 3 more jacks to his hand. Absolute complete run-ender. I didn't get any add or remove in this run, so I'm dead

[HeraldOfOpera]

If you were vulnerable to this kind of strategy, you died against the boss two levels ago. This is intentional.

[brickey.8]

Unless you can sabotage his cards, locking the jackpot isn't much different than just generating a few random jacks and repeating them every time he shuffles. It just makes him slightly more variance and more resilient. You should be able to survive an onslaught of 20s.

Now, if the enemy who has a 2 mana 4/7 locks that? That one is pretty much guaranteed game over unless you can burn or unlock it.

[Orilune]

Yeah, there's a reason I pick up any "Gerald from Riviera" or Yellow Card that I can get, even if it's just because I'm running a Heart-heavy deck and don't feel like dealing with Alucard's nonsense.
Killer Queen sorta qualifies too, but I'm not a fan of cards that can instantly bust me from a 10, so I tend to avoid that one.

Other options to mess with your opponents would be Jumping Jack/Queen of Chess, both cards can switch sides, although JJ can come back to bite you very easily, and 2/4 "Terminate the Tower" options can also mess with these strategies - doubling an opponent's card's value easily causes a Bust, and doubling anything above 10 just straight-up gives them an instant turn-ender too.
Messing with their limit can be dangerous - putting the enemy you found at Max:20 would mean tons of Blackjack effects too, but anyone running a 21 or any strategy that's finely tuned to hit 21 with several big cards also gets thrown off hard.
Meanwhile, the Rules Card also does the same things Sweep the Leg does, but this time you get to choose what happens - only to one player though.
The best part is though that most of these cards are unlikely to bust you because you can either make them go away or they aren't worth that much to begin with, so they should fit into most decks that aren't already in a state where you end up at 21 no matter what.

Two more disruptive cards would be the Dis-Card, which is just a temporary solution, but still valuable, and the Business Card, which can be very useful, but is more likely to be a massive pain in the neck because your opponent can play it right back at you until both decks are completely dismantled.

Anyways, good luck, and if you find that you really can't deal with that enemy, you can always go down the other path until you find something that works for you :)

[brickey.8]

Killer queen is actually pretty safe to use, at least if you have a big deck that can afford to lose a few of its big-number cards. Anytime she would bust you, you can just target one of your own cards instead of the opponent's. You only actually bust if 12 minus your highest other card would bust you, which is rare.