Tone Back the Might of Demacia
Let's start this off by saying that I appreciate Garen as a champion. The concept behind him is solid, and he has the potential to be a truly great champion. However, calling him "fair and balanced" is like calling Fox News the same thing. Simply not true.
The problem with Garen is that he excels in too many areas. With one item (Black Cleaver), he becomes a powerhouse. This isn't the problem. The problem is that he lacks any other incentive to build attack damage. He can build straight into a tank build after this item (which also provides health), and doesn't fall off in terms of damage because of this choice. In addition, his Q not only provides him with a nice damage and speed boost, but it takes away counter play by silencing for 1.5s. In this time, he is able to start spinning (and therefore winning) before the enemy is able to start fighting back. But they can't duel him, because he has far more health and much higher resistances. And then his ult serves as his coup de grace, executing the enemy while he dashes off quickly afterwards, largely intact. If he were an assassin, with a significant risk should he not land his full combo, this would make sense. But he's a tank freight train barreling toward your carries with the might of a thousand suns behind him.
Even in lane, he poses a significant threat, because he doesn't need to go back nearly as often as other champions, because he gets free healing from his passive. He loses a lot of risk from duels, because should he lose, he simply needs to wait it out until he's back to his healthy self. You can try to deny him farm, but his silence quickly follows these attempts, after which you witness your health decline as his sword spins through you repeatedly. And you cannot escape this, because first you are silenced, and then he gains bonus movement speed during the spin, so he is able to stick to you for the duration of the spin.
TL;DR : Fix
. Pls.