Are you doing quick play or constructed? I do pretty much quick play exclusively, and it is exceedingly rare for me to go ten games without seeing merfolk, and often it’ll be every two or three games that they pop up. If you don’t nip them in the bud immediately, they just go off on you, like 5 or 6 3/3 or bigger creatures (at least one of whom is unblockable) by turn 4 go off.
EDIT: I encounter the same few decks time after time, too, it’s a little bit boring. If it’s not merfolk, it’s the black/green saproling deck, and if it’s not that, it’s the red/white deck that facerolls if anything faster and harder than merfolk. If you’re not lucky or incredibly aggro, you probably won’t win. I’ve taken to playing a white/blue Psychic Corrosion deck because I’ve always been a huge sucker for milling as a victory mechanic, but even with a deck full of counters and whatever cards are called that prevent creatures from attacking, I still get my ass handed to me plenty by decks that are just too speedy to counteract.