Slay The Spire

There are a few ways to get an infinite combo going, and all of them rely on some luck in having the right cards come up either in reward screens or in the store, and on getting events that allow you to thin out your deck. I don't think there's a way to guarantee this every time I'm afraid. You're going to have to play quite a few games until the right things fall into place - but you can improve your chances significantly by knowing what you're looking for. So I'm going to set out the way that I did it, which I think is probably most straight forward (although quite luck dependant), and a couple of other possible options:

The way that I did this was with Ironclad on Ascension level 1 (more elites so I could pick up relics, but not so difficult that I died while getting the deck set up). Your key card is Flash of Steel (Deal 3 damage and Draw 1 card) which is colourless. With Dual wield (Copy an Attack or Power card) you can copy your Flash of Steel at which point if you have a low enough number of cards in your deck you'll just keep cycling through.

If you see a True Grit (Gain block and Exhaust a Card) and can upgrade it so the Exhaust is selectable not random then that allows you to Exhaust other cards to thin your hand down during a fight.

Ideally you won't take any cards that don't fit in with this strategy, although just passing on rewards screens and relying on your starting deck for too long is likely to get you killed so if you don't see those key cards early, you may find you want to grab one or two good cards if they come up just for the sake of survival while you keep looking. Alongside that try to use the Stores and Events to remove cards that aren't playing an important role in your deck (probably Strikes and Defends). Once you have the key cards its all about removing cards.

Now, in your fights you want to use True Grit to Exhaust anything other than Flash of Steel and Dual wield, and use Dual Wield to copy Flash of Steel. It may take a few rounds depending on the size of your deck but at some point you should be able to get to a place where you've copied enough Flash of Steels that you play one, and it draws another, ad infinitum.

I had Flash of Steel come up in my first store, which I think was pretty lucky. If you're just going for this achievement you may want to just rush to the first store and restart if its not there. Or continue but switch to trying for other achievements at that point.

Pick up relics that help if you can. Extra energy from boss relics is helpful as it gives you a bit longer to get the combo going. I picked up a Happy Flower (Gain 1 energy every 3 turns). Sundial is similar (2 energy every 3 times you shuffle your deck). Runic pyramid (don't discard your hand) would be helpful in keeping the key cards in your hand. Anything that draws cards (e.g. Runic Cube) is good. And you've obviously got to survive for a few levels while you're getting all of this working so anything that gives you strength, dexterity, healing etc is going to help with that.

And that's it really, just be patient and keep trying. As with a few achievements in this game, you need a few things to fall into place, so if it doesn't happen for you in a run you'll just have to try it again.

The other loop that is available to Ironclad reasonably easily is Dropkick (5 Damage, If enemy is vulnerable gain energy and draw a card). Now the beauty of this is that Ironclad has Bash in its starting hand which applies vulnerable to an enemy. So if you see a Dropkick early this might be the way to go. Thunderclap and Shockwave apply vulnerable to multiple enemies so may be worth picking up here too, but be aware of your deck size. Then, as above, its about thinning down the deck so that you can get an enemy vulnerable and cycle Dropkick.

True Grit and Dual Wield are helpful here in the same way that they were above.

A third option is Pommel Strike (Deal 9 damage, draw a card) with Madness (make a card cost 0). You have to be lucky for Madness to target Pommel Strike (or manage it so they're the only cards in your hand), but when it does you have a card that is effectively equivalent to Flash of Steel in my first strategy.

Other classes have ways of going infinite too (example - Silent has Heel Hook which is similar to Dropkick but with Weak instead of Vulnerable). There are others that are more advanced but for me your best bet is to aim for one of the above on an Ironclad run and cross your fingers. Good luck!

With the Watcher, this can be done very easily with one card - Study
Below is my deck when I got this achievement. I found study early on in a game so was able to do this against a weak enemy.
Study gives you a zero cost card every turn called Insight. Insight is retained in your hand and draws two cards for free. All you need is a way to survive long enough to get 25 of these cards in your hand. For me, that was to use the power card Like Water which gave me 5 block every turn I was in calm stance, and versus one of the little bugs in the first area, 5 was the most damage it could do. You will either need a way to block each turn like this, or fight something that does little enough damage that you can tank the hits until you have enough cards.

I skipped my turn until I had a full hand of insights and 15-20 in my draw pile then spammed Insight cards until the achievement popped. If you drawvany other cards in your hand, try to use them to make space for insight, it is possible to draw a full hand of other cards and not have any insights left make sure you draw enough to get the achievement, for this reason, I recommend taking turns to get more than 25 of these cards.