He's strong because he does 50% Magic and 50% Physical. With a BotRK, you're essentially doing 8% of their current in physical to murder them down then finishing them with an 8% of missing as magic. Early on, most don't know to get a Banshee's or Spirit Visage THEN build into armor since he needs his Trinity first, which means he'll be reliant on his magic damage bases to do most of his damage for him. After he has the Triforce, you'd build into a Randuin's or a Frozen Heart since you need big armor numbers and reducing his AS so he's not shredding you by repeatedly AA'ing you. Just gotta go some one tanky who's got good sustain and possibly an HP steroid. Champions that you DON'T want to get in their face are usually ones who would destroy a Fizz. Akali, Viktor, Darius, Renekton.... All those huge bursters who can insta-gib or not care if you jump them.