This might well make you smile, but it's 100% real. There is a summoner by the name of 'Riste' who one-tricks Garen. The American player has piled up nearly 4.6 million mastery points on The Might of Demacia, and just hit Challenger.
Although he joined Summoner's Rift in Season 1, it was not until Season 9 that Riste managed to reach Challenger — accumulating more than 700 games on Garen in the process. Overall, he has a win rate of about 52% on his favorite champion.
For the curious little ones, Riste has a Twitch and YouTube channel, where he reveals some interesting mechanics to his viewers to help them perfect their mastery of the champion, such as the hidden mark.
By reaching Level 6, Garen designates a Villain. If Garen then uses his ultimate on him, the damage he does is monstrous. That's why Riste advises keeping hold of your skill point when you reach Level 6, until you engage in a fight. Picking up your ultimate skill at the last second will surprise your newly designated Villain, inflicting way more damage on them than they were expecting.
It certainly reminds us of Annie's hidden stun, so we'll incidentally give you this trick. Annie is able to stun champions with her spells (Q, W & R) when she reaches four stacks on her passive. Each stack is gained by casting a spell, and fills a gauge just under her health bar. Your opponent can see that gauge, and most of the time they will flee as soon as you hit the fourth gauge.
To counter that behavior, you need to reach 3 stacks. Keep your Molten Shield (E) up while rushing at your opponent, then cast your Disintegrate fireball (Q). During the flight of the spell, quickly press E. Your fireball is suddenly capable of stunning.
So, either Garen or Annie — if you want to climb the ladder... you know what to do.