itch.io

Game Maker has support for networking, but I've never managed to get online multiplayer to work myself so I'm afraid I can't offer much help here.

Engine-wise I'm thinking you'd have the other player act like an AI player, and then their moves are based on what the player selects, with netcode synchronizing player inputs... you'll also need to make the RNG shared so things like variable damage and chance-based effects will be the same for both players.