6 items master yi vs 6 items shyvana with new devourer

if shyv has botrk/sated/tabi/randuins/thorn/hydra or something like that shyv would win 9/10 times

Shyv. Too tanky while still dealing a lot of damage. If Shyv gets thormail, then there's no question.

Shyv is a great 1v1 does mixed damage, and can build tanky but still dish out a lot. even though yi would do true damage he cant sustain a thornmail or her bite >:3

Are we assuming they are robots that just stand right next to each other and attack until one of them dies?

Or is Yi allowed to run away for the duration of Shy's ult and only return to fight her when it is down again?

Really depends on what Shyv builds and the conditions of the fight arena.

6 item ADCs are pretty terrifying and pack AP Carry burst-power in auto-attacks alone, auto-attacks that cut through defenses like butter. 3 item ADCs? You can easily kill those with Thornmail shenanigans. Full build is a completely different ball-park: defenses atrophy in scaling potential late, and where Shyvana is spread between needing to itemize for offense and defense: Yi really only need to go into offense, gaining a ton of exponential scaling with AD * AS * Crit * Crit damage * armor penetration * life steal. With 6 items - Yi could also easily grab a Guardian Angel, cutting back on a decent amount of Shyv's offense and really screwing her over if she's to reliant on her ult.

It's also worth noting Shyvana's primary strategy lacks a certain amount of finesse. It's essentially charge, crush puny mortals. Yi has the capacity to play much smarter - utilizing some pretty painful poke tactics with Alpha and ending on a nearer minion, he can also choose to evade key Shyv skills with Alpha, and he could activate Highlander to wait out Shyvana's ultimate and run away to utilize Meditate to recover. Yi without his ultimate is still incredibly strong late-game: Shyvana not nearly as much, her QWE skills are all fairly weak.

Ultimatly I see this as sort of a Mario vs. Bowser scenario. Shyvana is big and tough and mean and will clearly win a fight where both try crushing each other. Yi, however, has a lot of tools to out-Teemo Shyvana and win through irritating tactics if he's played by someone smart enough.

Yi would eat her. I've seen a 5 item Yi kill a Rammus with every single armor item before. Yi's late game is dumb.

tbh it's really from itemization. if they both take challenging smite and both have bork i don't really have an answer.