Deny Garen’s Passive by auto-attacking him whenever possible. This will reduce his sustain in lane and make killing him easier.
Garen’s E deals extra damage to lone targets. You can mitigate the damage by fighting beside your minions. They will also start focusing him if he auto-attacks you.
Try and stay at max distance at all times against Garen. This will reduce his all-in potential. Additionally, by standing at the maximum distance possible, he will have to waste his Q to get in range of you to trade. You can capitalise on this and look to trade with him while it’s on cooldown.
Garen’s build path can be very different from game to game. Keep an eye on his itemisation and play accordingly depending on what he is buying. If he is going full damage, you probably won’t be able to fight him alone.
It will be difficult for Garen to engage or get on to carries in a team fight as he lacks hard CC. This means that he will try to flank in team fights with his Q. Make sure you ward to the side of you at all times and kite backwards as soon as he appears.
Avoid wasting valuable resources like Summoner Spells or Ultimates on Garen unless you’re certain you’re able to kill him. Garen can quickly top up his health bar back to full thanks to his Passive which will result in your team losing out on key spells for the remaining of the fight.
Garen’s kill pressure in the early game can be difficult to handle if you’re playing as a tank. Respect his all-in and try and avoid trading with him unless his Q is down.
At level 6, Garen will gain access to his Ultimate R which can be used to execute enemies. Do not fight him when low, and definitely recall if you’re below 50% health as he might look for a trade. It’s better to be safe rather than try and fight and give him a kill.
Once Garen has completed his first item, he will be more obnoxious and looking to trade more and more. Keep this in mind and do not commit to fights with him alone.