Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Early Game- go for first blood any early kills, lane w/ a hero that harasses well.
preferred lane partners - Blitz, Shen, Morgana, Soraka
heroes that are a pain - Mord, Amumu, High hp heroes (Garens attacks require close range combat, if you cant deal enough dmg in a short period you will have a lot of trouble)
Mid Game - farm when possible, its very easy to build up courage w/ his judgement/strike
Gank as often when you can with ghost/exhaust, its extremely easy to tower dive w/ strike, jugement, courage (reducing most dmg from tower), and for the kill w/ justice.
Late Game - with mallet you can aoe slow everyone while being able to tank with courage. Silence when possible, a hybrid Garen Build will give enough dmg to carry but also tank.
Rune- Armor Penetration for extra dmg w/ judgement and strike, also helps w/ farming
Crit Dmg - Jugement can Crit, any xtra crit dmg will help your dmg output
Magic Resist - Helps for taking hits, most likely you will be on the front line being that all of garren skills are melee
Masteries - Extra dmg is only necessary if you have courage + mallet
Spells - Ghost + Exhaust + Garen skills = Auto Kill
Conclusion - This build is based on a hybrid Build that relies on his AOE Slow (mallet + judgement) and damage output from Ghost. <- the 2 most important items
** if playing against multiple physical damage characters
-go mass sunfire capes
it will also allow easy farming