Garen simply can't carry if the enemy team has any form of competence whatsoever and your team is sub par. Here are your winning conditions.
- If they let you split and your team doesn't engage, they lose.
- If they answer your split and your team is good, they lose.
- If you run into the enemy team and don't get CC locked, you can kill a key member with QER. They lose.
- If they send their entire team after you and either you escape or your team gets something, they lose.
Meanwhile, here are your main loss conditions.
- You can't 1v1 the enemy toplaner late (or whoever else they send to match you). You lose.
- You can't 1v2 the enemy toplaner & jungle late (or whatever pair they send to match you). You lose.
- The enemy team has good waveclear and you can't even engage in the first place. You lose.
- The enemy is a picker team. You lose.
- Your other lanes are doing poorly. You lose.
- Your team has no macro knowledge and waffles about while you split. You lose.
- They ignore your split and push faster than you and take more objectives. You lose.
- You lose lane AT ALL. You lose.
Those loss conditions are WAY easier to run into than it is to achieve your win conditions. It's not reasonable, nor a good use of time, to play Garen expecting to hypercarry. All it takes to beat him is a firm grasp of macro knowledge.
If your heart is set on him, I'd recommend the AS/offtank bs that's been popular lately. You rush Triforce into tanks/teamfight build or Sanguine Blade into squishies/dedicated split build, get a Phantom Dancer, DMP, Sterak's, Malmortius and just QE the enemy to death while never dying. Mercs or Tabis depending on team comp. Runes are Precision primary: Conq, Triumph, Legend: Alacrity (or Legend: Tenacity if they have lots of CC) and Coup de Grace. Resolve secondary: Conditioning and Overgrowth (or, again, Unflinching if they have lots of CC). Stats are +9 Force, +9 Force, +10 Scaling Health. Take Ignite if you're confident you can kill early and snowball, otherwise take TP.
