You know, for a skirmisher, Tryndamere isn't very good at penetrating durability.
Unlike Assassins, Skirmishers aim to shred through any nearby enemy that approaches. Because Skirmishers lack high-end burst damage or reliable ways of closing in on high-priority targets, they are instead armed with situationally powerful defensive tools to survive in the fray, along with extreme sustained damage to cut down even the most durable targets.
Tryndamere's total damage output is more strongly mitigated by the tank auto-%-reducers than any other champion in the game. Things like ninja-tabi, frozen heart and randuins omen are extraordinarily effective against Tryndamere because his damage is almost exclusively critical strikes-- and that is fine as he stands in his current state.
But if he can't "cut down the most durable targets" (aka, those that build those items) his current state isn't really skirmisher-like is it?
Hell for that matter, if he has the most health at level 1 and second most health at level 18, he isn't really skirmisher-like is he? What the hell is an aoe 80 attack damage debuff doing on a skirmisher? It hardly fits his theme, and certainly doesn't do anything to help him realize his agenda.
He's got so much power in odd places to help shore up his flaws its ridiculous. He ends up split pushing because his kit is incoherent, so the only way he can be effective is to leverage his raw numbers to overpower lone enemy champions in a side lane.
His damage to tanks is too low and his damage to everything else is too high.
You can't make a champion who's job is exclusively to split push, because the lack of versatility means they have to absolutely dominate that one task to the extent that it isn't possible to counteract it. In other words, if Tryndamere can be countered in a split push, he's garbage, if he can't he's unstoppable.
The only way Tryndamere can be in a healthy spot gamewise is if he has comparable versatility to the other members of his class. He should have some agency in a team fight, and that means he needs to impose a threat against champions who are nearby, even the tanky ones, the same way that gangplank, yi, fiora and yasuo do with their armor pen and true damage.
Right now Trynd is tuned by just overloading him with absurd numbers, raw damage, health, ridiculous power in mocking shout, and so forth. I think his numbers should be reduced across the board and in exchange he should hit tanks hard. Really fucking hard.
His target selection isn't even half as good as yi, fioras or ganplanks-- Yi has slow immunity, fiora can dash every 2 seconds and has an enormous amount of move speed during her ult, and gangplank has ranged attacks every so often. We have enough skirmishers who can run people down-- who have a stint of assassin like qualities to them.
I think Trynd should be more of a defensive skirmisher. His abilities should help him survive focus fire, and his job should be to wack things that are pinned near by, not try to dive like olaf into the middle of the enemy team, and certainly not split push all day because he has no other choice.
Mocking shout should be replaced with something that actually helps protect Trynd from focus, not something that helps him chase or "removes 80 attack damage". Like if he brought his sword up and blocked damage targeted only at himself from one direction (like a selfish braum shield), with a tiny cooldown--- something like that would make Tryndamere less binary because he wouldn't be forced to dive 24/7 to offer effectiveness to his team.
TLDR Basically, Trynd needs to offer similar versatility to other skirmishers in order to be healthy for the game, be more defensive and less sticky than the other skirmishers in order to differentiate from them, he needs a proper defensive ability on W, his numbers need to be adjusted so that they aren't so damn high, and his damage needs to be lower and more neutral.