According to the search results, Guzzlord is a Dark and Dragon type Ultra Beast in Pokemon Go. It is weak against the following types:

• Fairy (double weakness)
• Fighting
• Bug
• Ice
• Dragon

Therefore, the best counters to use against Guzzlord are Pokemon with these types of moves. Some recommended counters include:

• Mega Gardevoir with Charm and Dazzling Gleam
• Shadow Gardevoir with Charm and Dazzling Gleam
• Shadow Granbull with Charm and Play Rough
• Mega Diancie with Diamond Storm and Moonblast
• Xerneas with Geomancy and Moonblast
• Mega Rayquaza with Dragon Tail and Outrage

It is also important to note that Guzzlord is only appearing in raids until November 23, 2023, and can be caught with the use of Premier Balls after defeating it in a raid battle.