A list of decks that i got 12 wins in classic challenges with, along with a brief explanation of how to play them. This list has been updated over the years, in chronological order. Because of how quickly the meta shifts in Clash Royale, not all of these decks are necessarily viable in the current meta.

• Log bait: knight, princess, goblin gang, goblin barrel, ice spirit, inferno tower, log, rocket. Bait out their log, then go hard with knight + goblin gang or knight + goblin barrel. 1-crown deck.
• Miner poison: miner, poison, knight, electro wizard, inferno tower, log, ice spirit, goblins / skeletons / bandit. Constant defense and counter-pushing. Miner chips. Bring out the poison in double elixir time. Never let them get through. 1-crown deck.
• Spell cycle: knight, ice wizard, ice spirit, skeletons, tesla, log, poison, rocket. Spell cycle from the very beginning. Win condition is rocket, support win conditions are log and poison. Knight, tesla and ice wizard are pure defense. Never let them get through. Win as many battles at the bridge as you can. Make it as hard as possible for them to cross the bridge. 1-crown deck. If you lose 1 tower, make sure it becomes an overtime game. In double elixir, rocket cycle like a madman.
• Golem hut: golem, goblin hut, flying machine, night witch, mega minion, royal ghost, poison, zap. Bank elixir and defend until 1:00 - 1:30, then go for a golem push. The hut functions like an elixir collector because it lets you bank elixir and forces them to respond to the trickle damage from the spear goblins. 1-crown deck, is quite strong in a 1-1 scenario.
• Recruit miner poison: spear goblins, goblin gang, bats, inferno dragon, miner, poison, zap, royal recruits. No longer viable after recruits were nerfed by increasing their elixir cost from 6 to 8.
• Hut ram: barbarian hut, valkyrie, battle ram, fire spirits, poison, log, flying machine, zappies. Three cards in your deck are poison bait. Proceed accordingly. In desperate moments, valk + battle ram spam at the bridge can win you the game.
• Giant skeleton mortar bait: giant skeleton, mortar, goblin gang, dart goblin, goblin barrel, skeleton army, bats, rocket. Enough bait to beat even triple-spell decks. Notice that this deck only has one spell, and it's a very expensive one at that. The ideal push is giant skeleton crossing the river with mortar behind it, and a goblin barrel in front. Don't be afraid to play very aggressively. Valkyrie shreds this deck.
• Pekka miner: pekka, royal ghost, bandit, ewiz, miner, poison, zap, minions. Combos: pekka + ewiz (the evergreen combo), bandit + minions (sudden bridge pressure, like Sergioramos did in Clash Worlds), pekka + miner (of course!), minions + miner, miner + poison (the poison is usually added in to finish off the tower). Starting off: cycle a miner onto the inner corner of the tower, which no one expects. This is a gamble because they might have tornado, so be aware of how common nado is in the meta. Hover zap in case they counter your miner with gang or skarmy. Always poison pumps. This deck has no 2 elixir or 1 elixir cycle cards. Zap is a very useful spell, so don't throw it away. Royal ghost cleans up rascals. In an emergency, miner can counter royal ghost or bandit on defense. Minions clean up valk pretty well, just don't play them too close to the bridge.
• Bandit bait: bandit, cannon cart, princess, goblin gang, goblin barrel, spear goblins, log, rocket. Pressure them with bandit when they are low on elixir.
• No-direct-damage-spell hyper bait: mega knight, ram rider, princess, goblin barrel, goblin gang, dart goblin, skeleton army, bats. Get damage early and then defend until you're blue in the face.
• RG sparky: royal giant, sparky, ice wizard, tornado, mega minion, lightning, barbarian barrel. Save the sparky for golems, of course. Play somewhat aggressively in single elixir. Even if your ice wizard has a sliver of health, consider placing a royal giant in front.
• RG furnace lightning: royal giant, electro wizard, lumberjack, baby dragon, mega minion, lightning, barbarian barrel. You usually play RG at the bridge after defending a push, and then lightning their defenses (in double elixir). Doesn't fare well against miner cycle decks.
• No-direct-damage-spell hyper bait: giant skeleton, cannon cart, mortar, princess, goblin barrel, goblin gang, skeleton army, bats. Unlike the other deck, this one doesn't have good defense so be more aggressive.
• Extreme fireball bait: barbarians, royal hogs, magic archer, zappies, bandit, baits, log, fireball. After barbarians got buffed from spawning 4 to spawning 5 while dying to fireball, they became bait cards. Treat it like any other bait deck - spread out your bait in both space and time.
• X-bow 3.1: xbow, tesla, knight, archers, fireball, skeletons, fire spirits, log. It's 2.9 with knight instead of ice golem, and fire spirits instead of ice spirits. Fire spirits do a much better job against minions and minion hordes than ice spirit. Because of the slower cycle, you will play less aggressively in single-elixir. Most other things remain the same.
• X-bow 3.0: xbow, tesla, knight, archers, fireball, skeletons, ice spirit, log. It's 2.9 with knight instead of ice golem. Aggressiveness level is between 2.9 and 3.1, as you'd expect.
• Pekka pure counter-push: pekka, cannon cart, ewiz, baby dragon, mega minion, tesla, poison, log. Wait until you're sure they're low on elixir, then slow-roll a pekka and barrel past their defenses. Until they're low on elixir, play non-stop defense. Baby dragon is the best card to cycle, unless they have bait. Cannon cart often takes the tower.
• X-bow pump: xbow, elixir collector, tesla, ice golem, log, skeletons, musketeer, snowball. If you do not pump aggressively, you will lose. This is a poison/fireball bait deck. Play accordingly. Your defense will resemble the defense for xbow 2.9 or 3.0, but with musketeer instead of fireball. Losing one tower is almost always fatal.
• Mega knight bait: mega knight, wall breakers, princess, goblin barrel, dart goblin, goblin gang, goblin hut, bats. This deck was meta at a time when wall-breakers, witch, elixir golem and night witch were overpowered. It has no direct damage, so you generally play aggressively, trying to bait out their small spells and then going in with wall breakers, which only cost 2 elixir and cannot be countered for a positive trade. Mega knight often takes the tower in this deck.
• Mega knight bait (again)
• X-bow 3.3: xbow, knight, musketeer, goblins, tesla, fireball, log, ice spirit. Basically beefed-up 2.9. Play more defensively in single elixir than you would with 3.0 or 2.9.
• Triple building miner poison: miner, poison, log, bats, guards, bomb tower, furnace, tesla. Great deck to counter egolem battle healer night witch. Lost to a logbait player who rocket cycled me. Not necessarily a great deck, but it counters bridgespam well.
• X-bow 3.0 again (2nd time)
• Splashyard: knight, ice wizard, baby dragon, tornado, barb barrel, graveyard, poison, goblin hut. Goblin hut got a death spawn of 3 goblins in this season - an absurd buff.
• Lava hut miner: lava hound, mega minion, baby dragon, flying machine, goblin hut, barbarians, miner, arrows. Triple fireball bait. No one expects barbarians in this meta, so use them as a surprise card.
• Double win-con logbait: knight, princess, dart goblin, skeleton barrel, goblin barrel, skeletons, bomb tower, log. 5 logbait cards with no big spell. Play aggressively. You generally have matchup against all cycle decks except xbow. I didn't face too many beatdown decks but I assume the strategy is to cycle back to another bomb tower to double-kite the golem. Lava hound is probably a hard counter.
• X-bow with knight musketeer: xbow, knight, musketeer, tesla, fireball, log, ice spirit, skeletons. This one is special to me because I did it with level 8 xbow and level 8 skeletons (and everything else level 9), on my mini-account.
• X-bow 3.0 again (3rd time), on my mini-account
• Golem clone: golem, night witch, flying machine, bats, skeleton barrel, goblin gang, arrows, clone. Only won my 11th and 12th matches with this. 12th match was against a counter - egolem / healer / heal spirit / skeleton dragons / edragon / tornado / barb hut / fireball. I got to 10-2 with other decks. Still, a fairly solid deck. Flying machine usually gets the most value. Track their splash cards.
• X-bow 3.0 again (4th time), on my mini-account
• Mega knight fireball bait spawner: mega knight, flying machine, cannon cart, night witch, goblin hut, furnace, fireball, barbarian barrel. Got to 11-0 with it, then 11-2, then used x-bow 3.0 for the 12th win. Lazy, uninspired deck.
• Mega knight fireball bait again, all 12
• Mega knight fireball bait again, all 12
• Mega knight fireball bait again, all 12
• 16-3 with mega knight fireball bait