Garen's regeneration greatly increases if he can avoid receiving damage for several seconds.
Judgment deals maximum damage when only hitting a single target. For effective trading, try to position such that only the enemy champion is hit.
Garen is only constrained by cooldowns, making items such as Black Cleaver very effective for him.
Stack armor items to lower the large amount of physical damage that Garen deals out.
Try to run away from Garen as your health gets lower, as he can execute you quickly with Demacian Justice.
Be careful about attacking Garen in brush. It often will lead to taking full damage from Judgment.
Judgment deals maximum damage when only hitting a single target. If getting out of its radius isn't possible, move through allied minions to reduce damage taken.