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The problem with Garen isn't entirely that he is "too strong" or "too weak." The problem is, with his skillset, he really can't be anything but stronger and tougher than other champs, or else he'll be useless, because of just how minimally useful all of his skills are for anything but damage.
Looking at his skills, we have...
A very, very short duration speed increase (it's not nearly as long as the duration of the buff on the attack) along with a somewhat more powerful melee attack that silences. This is seriously the most contribution Garen brings to his team besides damage: A mediocre duration, mediocre CD, melee range silence.
A passive armor and MR increase with an active damage reduction. While shields are generally pretty good, all the "shield" type moves I can think of are more interesting than that; Morde's provides AoE damage and charges his shield (and can be cast on allies), Udyr and Shen's are both spammable and Udyr has more effects besides just the shield on that skill, Galio's lets him self heal, etc.
An AoE damage move with some minor anti CC effects. It's seriously just AoE damage over time.
A purely damaging ult; it's just used to finish people off. Yeah, it's unique in that it does damage based on how much HP the enemy is missing, countering HP stacking (though doing true damage might be more interesting, since TBH resist stacking is far more prevalent and effective than HP stacking), but really, it's like Cho'Gath's feast if you had to use it to take kills away from better scaling enemies, it didn't have an additional effect, and it's base damage was lower.
Long story short: Garen brings absolutely no form of CC or utility to his team. None of his skills affect allies, his silence is incredibly marginal (not terrible, but just... marginally useful if you can stop a channeled ult with it), and the rest of it is purely damage, all at melee range. The only way Garen, in his current state, fits into a team is if his base damage or scaling is simply better than everybody else's. That's why I think Garen, as he currently is, can't really be "balanced;" he's either going to be picked and complained about because his base survivability and damage is better than others (pre-patch Garen) or he's going to be set up so that his base stats and scaling are equal to or worse than other heroes, and he's going to be useless since every other hero provides more team utility in some way, shape, or form.
I'm not saying Garen is boring. He's certainly fun to play when he gets on a roll. But it just feels like he needs to get some skills tweaked in order to be useful, while still being fun to lane against and being able to bring something to the team besides just good survivability and high base damage.