User blog comment:Chase999/Anti-meta Champions/@comment-5035765-20140219070330

actually what you're asking for is what beats certain Flavour of the Month champions and strategies

which varies by champion, since everyone has a different counter, pretty much...

against the two you mentioned (Yi and Tryndamere), champions that benefit from high armour, or naturally have high armour because of their kit are pretty good against them (Rammus, Taric, Leona, Malphite, or a Thresh that's good at getting souls, off the top of my head). Also, since those two tend to depend on AAs, items with AS slows or that punish AAs are good to build against them (Thornmail, Frozen Heart, Randuins... all of which also have high armour). Finally, Melee carries like those two have a well-known vulnerability to slows and other forms of CC, so any champ with CC is your friend, as well as any items that grant you CC (Rylai's Crystal Scepter, Frozen Mallet, and Iceborn Gauntlet are all nice for this, though they're all QUITE expensive)

Countering is all about Analysis + Reaction. Figure out what the other dude is using and make that thing not as good by building against it.