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.