I'm going to be short and clear. Why critical and armor penetrarion runes? Judgement is based on physical dmg and it has greter dmg with critical chance. So 25 armor penetration and 8.3 critical chance will pwn anything on ur way especially frigides.
* note instead of and health potion you can buy but I personaly prefer first combination because of good regen balance at the beggining.
*note - always cast hit the enemy and than cast

It is important to try to have and before 20 minutes. I usually have that in 12-18 min which is extra important because once u get this you are on good way. Why? Having 2000 hp and dealing enormus damage while Casting Judgment will enable you to her*** more than before enemys and gank them so easy.If you don't believe try and convience yourself.

*note - after Sunfire Cape and Boots of Swiftness you can choose between or . So what to buy and why?
Firstly if you feel like you are doing well or you are just pro with this champ like me than you will buy . Why? Easy to answer. Once you have all 20 stacks you can consider yourself nearly immortal(15% dmg reduction + your natural high armor and magic resistence combined with items - pricless(I didn't mentioned :) ).True story.
Secondly, is not that bad option too.Why? It is sick item so after having sunfire cape u will have assists and kills for sure and brutalizer is only 1337 gold. This will reinfore your her***ing becaue you will penetrate 40 armor, have your cd reduced and have 25 attack dmg which helps a lot when playing Garen.

After buying this early game items we move to mid game ones. It all depends against who you are playing. After buying items previously mentioned I buy . Why? Because you will have nearly 4000 hp with some stacks so you will dominating around terrain.
*note - If the champions are nuke based than buy
*note - Buy simple as that.
Last two items you can buy if you feel it is really needed. If not, instead of that buy one more or

Final Item Dont ask why becaue you have no rights to question my knowledge =)
*note - about this variations of items which I mentioned you should have feeling
* note - In Late game sell your brutalizer(because it won't be needed anymore) or Leviathan( if you suck )

This perfectly works for me. I usually have 7:3 (wins:defeats) with Garen.
If you have any question feel free to ask.
Fight for Demacia with this build.This is one of the best way to obey your duty which Garen pledged to Demacia!!!