Tahm Kench is a water type pokemon, so electric attacks (Kennen, Xerath) are supereffective!
Champions with HP% damage or knock back ability are effective against him. Try to pick Vayne, Gnar or Alistar against him. Blade of the ruined king is also a good item to counter him.
Percentage health shred or items such as Blade of the Ruined King and Liandry’s Torment are very effective against Tahm Kench. His ultimate’s passive grants him bonus damage on attacks based on his bonus health, so he won’t stack resistances.
If he eats ally with low health and u have champion with global ult (Ezreal, Draven etc) wait a proper time before you ult in front of him.