Garen Guide

Tips for Playing Against Garen

-This one feels more difficult than it used to be to me. I would just be careful early and more useful later if you have a good team for dealing with his split push.
-Use E a lot during trades with him so he can't get you low enough to be in ult range, but also I would avoid trading with him unless you're ahead, as he can sit back and heal it up often
-Q and if he runs up to Q you, and wait his Q out. If he gets onto you, at least you could get a Q grasp in and maybe even avoid the spin damage.
-Remember that his W gives him a lot of tenacity, so you might not be able to get a QW stun combo onto him reliably.

Bami's Cinder

You could go for a Bami's item, such as Hollow or Sunfire! I would do this second or third item if needed, but the idea is that you stop tower bleed and uneven teamfights from his insane split-pushing and roaming.
Heartsteel

Heartsteel is just great in general, and if you're ahead you'll probably destroy him with it! Aside from that, the extra range is great for escaping his Q, or even chasing him.
...

Um. Idk. Iceborn or Dead Man's maybe? Probably depends on the rest of the enemies or allies.