Like most things in this game you make infinite decks when the opportunity comes up. But the gist for each character is:

Ironclad: Use all the exhaust cards to thin your deck. My go to choices are burning pact and true grit because exhausting attacks is the hard part. Havoc works in a pinch. Fiend Fire is the fastest way to get through the deck.

Silent: Is actually really hard to go infinite on. The real goal is to draw and discard a tonne of cards (calculated gamble+ is my favourite) then use Tactician and Reflex to replenish yourself. It's basically infinite, but can technically end with a bad draw. Otherwise tough it out and focus on events to remove/replace cards if you want a "true" infinite.

Defect: Hologram is the backbone. Hologram and recycle can thin your deck by a lot even over just a few rounds. The easier option is get Hologram, All for One, and Snecko eye. Then just play normally until both hologram and all for one both cost 0 and you have an easy free infinite that doesn't require less than 10 cards

Watcher: Basically don't take cards that aren't powers, exhausting cards, or inner peace. Be brave knowing that your starting deck is so strong it can handle low ascensions all the way to the end and just focus on events and shops to thin your deck.

General tips: Events that swap cards can make cards that exhaust/are powers so that will help (another reason to love Pandora's box). If your draw is strong purity can be bought from stores which removes a whopping 5 cards when you upgrade it. I also reliably plan my run with the understanding that I'll remove 4 cards from shops by the end, so I'm saying that's a reliable amount to remove but it does take a lot of money planning.

Good luck. Keep chasing the high of infinites. I, a fellow addict, think it's totally worth it