Hey Ring2020 here! Just wanted to say that because Tahm Kench is a meta for mid it can be a high cost high reward builds. First off, not all champions have been tested with TK in mid yet. There may be others, but this is the general important champs against TK. That's all for intros and hope you have a good time looking through this.

You need some penetration, Attack speed because of stacks. Scaling M.R. and Armor for sustain. And Experience to be a bigger level faster.

First off, you needfor Tahm Kench because he can sometimes need an escape route. All champs do. You may want for a) a sneaky start and b) slow enemies for stacks to swallow enemies. You can also have the choice of for a spell. But that does give your start away some. Unless they think your top.

Pro: You get a sneaky start (does not work well enough depending on support and if you switched exhaust for ignite.) Assassins work less on you because you are considered a tank. You have a ton of magic damage capability, with speed from items, you can take down squishier mid-laners. Tons of poke potential. Especially with tongue lash and swallow on minion combo. (It's where you tongue lash a minion and right before you hit it, you hit swallow. Then spit at enemy.) You have your ultimate where you can tp back, grab items, then ulti to mid-lane. If you have enough health and you know enemy jungler is too far then you can kill a low health champ in their turret then e to save yourself.
Con: Sometimes you get deadlocks if you can't defeat the enemy easily. So you will get ganked a bunch of times for them to push mid. Enemies who deal overtime damage can out sustain you sometimes with the right technique and poke potential. If you start roaming, to help other lanes out, if you get ganked by someone with high speed, big attack, and has a flash you will die.

Remember when I wrote about the tongue lash and swallow technique (TLS technique)? Well with it you can poke at champs who are keeping you in your turret or you are keeping them in their turret. Your ultimate is a possible gank potentially (works better if you meet jungler in jg and cut the enemy off in the back). You can swallow champs and take them to your turret (slowly) then get them to take damage (Only works with low numbers of enemy minions in turret range, like 1 or 2). If you have 3 stacks then tongue lash the enemy, they will stun (If you have low attack speed you can only hit them once again and then they will unstun. So swallow them after a hit for low level and 2 hits with high attack speed). If you are almost dead and you have grey health hit your e (Depending on gray health you get that much shield which quickly goes away). You can also swallow teammates to save them or push them to weak enemy champs.

You can tongue lash enemy champs for more gold (If you have Start #1 right away). You can use the TLS technique to hit other champs (See Unique Skills on this). You can wave clear minions with your swallow ability (get a higher health minion for swallowing, that minion gets most damage). With high attack speed you can wave clear minion really quickly.

Later in game, use your ultimate to gank (with someone in you) a lone champ or a fight that is already happening against some other champs. You can also use the ultimate to get someone out of the fight (if they are quick and smart enough.) You can swallow teammates to save them or push them to weak enemy champs. Tonque Lash slows an enemy to where running away from or towards them is easier (works better with).