With the arrival of new Pokemon in Pokemon Go, the metagame surrounding gym control has dramatically shifted. While there are a few new attackers to help players quickly defeat gyms, the biggest change is the addition of Blissey as the new elite gym defender. Blissey has become a insurmountable obstacle for many players not because of its vicious offense, but rather its ability to absorb attacks thanks to its gaudy HP stats. As a Normal Type Pokemon, Blissey has only one weakness, which means that otherwise versatile attackers could come up short when trying to topple it in a gym.
The best way to defeat Blissey is to bring in a Fighting Type Pokemon or a Pokemon with Fighting Type moves as a specialist. Blissey's sole weakness are Fighting Types, so a strong charge move should really whittle down Blissey's HP. The strongest Fighting Pokemon in Pokemon Go are Machamp and Heracross, although any Fighting Pokemon will do provided it has a strong set of moves. In order to maximize your chances, try to bring in a Pokemon with the moveset Counter/Close Combat or Counter/Dynamic Punch to dish out the most damage per second.
Blissey will become even more common in gyms moving forward, so it's important to plan ahead and build up a strong counter now. Let us know what your best anti-Blissey strategies are in the comments below!