Efficient early lane trades and kill secure with [R].
Durability and crowd control
Every 4s your next attack on a champion deals bonus damage, heals you, and permanently increases your health.
After taking damage from an enemy champion, the next 3 spells or attacks you receive deal less damage.
After 10 minutes gain 8 Armor and 8 Magic Resist and increase your total Armor and Magic Resist by 5%.
Increase your Tenacity and Slow Resistance for each summoner spell on cooldown. Immediately after casting a Summoner Spell, gain a larger burst of Tenacity and Slow Resistance for a moderate duration.
15-135 health (at levels 1-18) and +9% attack speed
Takedowns on enemies grant permanent Tenacity.
Deal 7% more damage to low health champions. Gain AD/AP after kills or assists.
Unfortunately Garen doesn't have any fantastic keystone choices, Grasp is just the best of the bad.
You should be able to proc Grasp relatively easily in melee matchups with [Q] and the bonus max health synergizes well with your [W].
Bone Plating massively empowers your ability to take aggressive trades early in lane while mitigation most incoming damage.
Conditioning allows you to scale into the mid game as a very threatening tank.
Strong AD Matchups: If playing into a strong AD matchup like Renekton or Camille, take Iron Skin.
Strong AP Matchups: If playing into a strong AP matchup like Rumble, take Mirror Shell.
The bonus tenacity and slow resistance will help you overcome Garen's biggest pitfall of getting kited by CC and slows.
With Unflinching you should be able to stick to your target for the kill much more easily.
Legend Tenacity allows you to stick to your target so that you'r able to continuously pumping out damage to your opponent.
Coup De Grace's execution nature synergizes very well with the execution damage pattern of Garen's [R], securing kills that may otherwise not be possible.