I haven't played Clash Royale in years, but have recently dipped my toe back into it and am having a lot of fun (despite really souring on Supercell's brand of f2p design).
I'm still getting caught up to speed on the new cards and modes that have been added to the game in my time away, but I've been finding a lot of success with decks that I'm building now, so I thought I'd made an updated list of good card synergies I've found. See below:
Witch and Skeleton Army
The Witch is one of my favorite cards in Clash Royale because she spawns skeletons. Using her plus a Skeleton Army just makes her more dangerous more quickly.
Valkyrie and Inferno Dragon
A great all-purpose defense. Inferno Dragon can melt down any meaty units from the sky while the Valkyrie and take down any smaller mob units.
Knight and Barbarian Barrel
The Knight is a cheap tank that can be buffed considerably if you use a Barbarian Barrel to flank anyone he's fighting.
Valkyrie and Hog Rider
An oldie but a goodie. If you place a Valkyrie in front of a Hog Rider, he can push her forward quickly, making for one heck of a speedy and dangerous attack.
Battle Healer and Elixr Golem
Elixr Golem is an absolute powerhouse at taking down towers given its cost, but the tradeoff is that you grant your opponent Elixr when it dies. So... what if it didn't die? That's where the Battle Healer comes in. Send those two down a lane together and you'll get tons of value and damage out of the combo.
Firecracker and Ice Golem
Firecrackers are incredible damage dealers that can stay on the battlefield for a long time provided they are well protected. Ice Golems are a great, cheap unit to place in the way of any units that might want to take your Firecrackers out.
Hunter and Skeletons
The perfect way to use a Hunter is to get it close to your enemies while not becoming a target. Skeletons are the absolute cheapest unit that can tie up enemies while your Hunter blasts away at them.
Bomb Tower and Royal Delivery
The most effective way to use Royal Delivery is on large clumps of enemies, but it can be hard to place and time your drop to get the max benefit. This is where the Bomb Tower comes in. Use it as a decoy and then follow up with a Royal Delivery drop to make sure you hit a huge clump of enemies without fail.