Tahm Kench: Devour and Slow Reduction.
Did some testing and as it turns out slow reduction is highly effective on him when he uses Devour on enemy champions. The swiftness mastery alone reduces the slow he suffers to 56% (Weird interaction since it's should only be 15% slow reduction off a 95% slow so it must have something to do with the slow cap) adding swiftness boots to that reduces that to a 50% slow(It also increases his speed so he is still much faster), and using an elixir of iron reduces the slow from Devour to 33%!!
The vid shows this in action.