Muk is a Pure Poison-type Pokemon meaning he only has 2 weaknesses, this combined with his high durability, makes him into quite an effective Tank.
Tips and strategies
Effective attack types
Effective attacks types against Pure Poison-type Pokemon Like Muk are uncommon. Only Ground and Psychic-type attacks are super effective against it. With Ground-type attacks being either hard to find or have weak damage, your best bet would be to use Psychic type attacks to easily defeat Muk.
Most of Muk's attacks are Poison-type, so choosing a Pokemon resistant to Poison would be recommended. Ground, Rock, Ghost, Steel and other Poison-type Pokemon are recommended. Rock/Ground-types are a common combination and double resistant to Poison making them the best choice, big plus if they also have a Ground-type attack.