Hmm.. So I've been playing Kayn, from SP to JG to Top lol..... I really really like Tank Kayn, below is a video. It's not like it's unkillable but I think I have much better results when I go tank with him. I've reached mastery 5, so I'm moving onto another champ, but I haven't lost top lane (which is not my main role) so far on ranked games (plat~dia).
Unless it's a ranged champ, the Q harass actually does quite a bit of dmg with grasp healing some HP plus my E, I just walk through walls for fun. The slow enables kiting, so like Q AA slow AA or just Q AA slow and run back before ur opponent or their creeps can hit u. My first item's a Sheen and I really like the dmg output, have been able to go into 1v2s, 1v3s and come out alive, especially after you turn into darkin and get heals from R. Though I've also seen a dmg SA 1 shot opponent carries but they die too fast IMO, with proper guarding.