In Pokémon Go, which Pokémon and movesets should I use to beat Blissey in the shortest time possible?

Since it's a normal-type, fighting-type moves will be super effective against Blissey which means they will do 1.4x damage. As normal isn't super-effective against any types, your defender's type doesn't matter very much (though having a fighting-type Pokémon will benefit from 20% additional damage due to STAB). Any high-CP Pokémon who can learn fighting-type moves (and has a high attack stat) is a good choice.

The best choices for this would be Machamp or Heracross with Counter and Close Combat, but Poliwrath with Dynamic Punch would also do quite well. After that, Primeape, Hitmonlee, or Hitmonchan would be your next best bet. If you don't have any of those at high CP, just go with your hardest-hitting Pokémon with a fighting-type move or a good moveset (assuming that it doesn't use ghost-type moves).