Strategy and Tactics

Strategy

Remember that your job in the end is to silence an enemy, deal your combo, and find someone to finish off with Demacian Justice(R). Garen is very straightforward and people fear him because they know what damage is to come. The most important thing to being a strong Garen player is mechanical prowess. What is mechanical skill? It's juking, fast clicking and reactions, animation canceling, and overall accuracy/precision with attacks.

Because animation canceling is vital to every champion in the game, I will go into further detail as to what it is and how to carry it out. Animation canceling, also known as orbwalking from oldschool DotA players, is when you don't allow your champion to carry out the full animation of an attack or ability. For example, Garen extends his sword out and when it's at the pinnacle of extension, the damage for his autoattack goes off. He then retracts his sword back towards his body, as the animators deemed it necessary to make the movement look fluid and natural. If you move your champion directly after the damage of your autoattack goes off, you stop your champion from carrying out the rest of the animation (retracting the sword). This is to your advantage, because while you are attacking your champion cannot move. Using animation canceling on your autoattacks will allow you to lose less distance with each attack on a fleeing enemy and overall make you more mobile.

The Laning Phase

If I could describe Garen's laning phase with one word it would probably be “volatile”. You're either going to go into extreme farm mode or completely obliterate the opposing lane with your early game damage. You want to lane with either a Taric for his stun or a Soraka for the lane sustainability. Garen is actually very weak in lane without one of these two support champions to help him in the early laning phase. Without Taric, he has no way to close the distance between him and his target. With his stun, you can likely land a stun + silence + spin combo in lane and kill someone early on. However, the only other alternative to this is to get Soraka. Soraka simply spams heals on Garen to keep him alive and farming.

Remember that the purpose of laning is always to farm, not to get kills. If you outfarm your laning opponents then you have won the lane and will be stronger than them in a team fight or small skirmish. Bottom lane melee champions such as Garen are typically at a disadvantage in lane because they will constantly be harassed and whittled down by ranged champions as they go in for last hits. You put yourself in a position vulnerable to enemy autoattacks when you move in to get last hits, which is something to keep in mind.

Strengths: Tankiness and Damage

Once again, Garen is a tanky assassin. You will run in, Decisive Strike(Q) your foe, Judgement(E) on top of them, and bring down the sweet sweet Demacian Justice(R) for the kill. Afterwards, as with almost all assassins, your resources will be depleted and you'll be waiting on your cooldowns to come back up. However, you have done your job. You have turned the fight, as soon as it started, into a 5v4 situation in favor of your team.

As with all assassins, you make the enemy feel helpless against you. While silenced, they are unable to Flash or cast their abilities on you to stop you from killing them. Garen's strength lies in the fact that after he is done killing someone, he is still able to run headfirst into his next target and kill them once his cooldowns are up as well. He is a beefy champion and able to take a lot of punishment before being brought down.

Weakness: Mobility

Probably the only reason Garen isn't picked very often competitively is because his mobility is so low. Even though Decisive Strike gives you a short movement speed boost, it's simply not enough. The way Garen closes distance is walking up to the target, which is not similar to most other bruiser-type champions. Most melee champions have a way to close the distance in a much quicker way, such as Xin Zhao's Audacious Charge, Poppy's Heroic Charge, and Lee Sin's Resonating Strike.

Garen is easily kited in this respect. Throwing a slow down on him will make him a melee champion that runs slower than the ranged champions attacking him. If the ranged champions simply keep their distance while they attack him, then Garen has no way to retaliate. The prime example of this would be Ashe – a champion that counters Garen simply because he has no way to get up to her. This leads into my next point – positioning as Garen is key because if you run blindly into the enemy team you should expect for them to avoid you and kite you around.

Positioning

Assassins should always enter the fight from the side. Garen can do this, but he also serves the role of tank. If possible, you should flank the enemy and run at them from the side because this allows you to more quickly reach your target and gives you the element of surprise. However, when your team is grouped up, they need you to be with them and in their front lines. This is when you rely on your teammates to help you carry out your job. They should be able to crowd control a squishy target for you to assassinate. If not, then you must simply run in, headfirst, and tank damage until the opposing team is out of cooldowns and you are free to run and kill whoever you choose.

In conclusion, you want to either enter the fight from the side and start it, or run in with your team and fight the enemy head-on until you find an opening to kill someone.

Tactics:

The Laning Phase Before Level 6

Your weakness in lane will always be your inability to close the distance between you and your target. If you're not laning with Taric you won't have any form of hard CC to catch up to your opponents. You can use brush to your advantage by hiding in it and coming out with a Decisive Strike(Q) when the enemy doesn't expect it. Other than that, last hit and keep your creep score up. You will be able to completely obliterate the enemy squishies during the mid and late-game, so your focus should be geared towards that.

The Laning Phase After Level 6

When you hit 6, you can potentially use your Flash and Ignite both to try to kill someone. Flash, silence with Decisive Strike(Q), deal your massive damage with Judgement(E) and finish them off with an Ignite and Demacian Justice(R). This combo should be able to bring down most champions but it comes down to a lot of number crunching and knowledge of your champion's limits. Garen will kill, for example, Ashe with this combination fairly easily. However, if you're up against Miss Fortune and she has Exhaust or a support who will stop you from dealing damage, then reconsider using your summoners this way.

You're at a clear advantage at level 6 if you can simply close the distance and spin on them. You only have to get someone to about 1/3 of their entire HP bar and you'll finish the rest with a single ability (your ultimate).

Garen's Ultimate Cannot Be Canceled Once Animation Starts:

Timing Courage Properly

You'll be a constant target of enemy focus, so you want to save your Courage(W) active for when you see the most damage incoming. However, during laning phase, if a tower shot or enemy autoattack is in midair, it's almost always worth it to simply pop it and negate 30 or 40 damage  every 30 seconds. Courage(W) is geared more towards team fights when you need to use it at the right moment when burst damage is coming in. It will make you extremely tanky if you learn how to predict enemy focus fire and burst damage.

Tips and Tricks:

Judgement Cancelling

At times, you'll know that an enemy ability such as Tristana's Buster Shot, Corki's Valkyrie, Janna's Monsoon, or Kassadin's Riftwalk is up. If you use your Decisive Strike(Q) and then immediately use Judgement(E) after, you'll be spinning for 3 seconds and the enemy will be silenced for 2.5 seconds. This will give them a .5 second window of time to Flash away or gain distance away from you in some way. You should cancel your Judgement(E) after 2.5 seconds and get your Demacian Justice(R) off before they can do so if you know that it's coming or if you're absolutely sure that after cancelling the last second of Judgement(E) will leave them low enough to still die to your ultimate.

Decisive Strike as a Double Autoattack Reset

Decisive Strike(Q) is a strange on-next-attack mechanic. Not only does it reset your autoattack timer, but it resets it for the attack subsequent to Decisive Strike(Q) as well. If you autoattack, Decisive Strike(Q), and then autoattack again, you'll have three surprisingly quick attacks packed into a short amount of time.

Flash + Decisive Strike

This is a great way to initiate fights for when your team is too hesitant. Flash + Silence in combination with your Frozen Mallet slow will render them unable to move quickly and unable to cast any abilities for 2.5 seconds. This is a great way to start a fight and perhaps quickly burst someone down before they can retaliate.

Synergies and Counters

Synergies

Garen synergizes very well with single-target crowd control. Taric is the prime example of a champion that brings Garen all he needs. A well-timed Dazzle will allow Garen to run up and follow up the stun with a silence, spin on top of them, and perhaps finish them off with a Demacian Justice(R). Since this is your bread-and-butter combination of abilities, any crowd control will work for this purpose.

For example, if Blitzcrank pulls an enemy and pops them up in the air with a Powerfist, Garen can keep them unable to cast by silencing and killing them. He basically just needs a way to get to the enemy, whether it's a slow such as Nunu's Ice Blast or a stun such as Sion's Cryptic Gaze.

Counters

Garen is countered by slows and crowd control. Because he lacks a quick distance-closer and this is his primary weakness, champions that would keep him at distance such as Vayne and Ashe stop him from doing his job. He is countered most effectively by basically any champion that can keep him consistently rooted, slowed, or stunned. Garen needs to be up in an enemy's face to do his job, and he's useless when kept at distance.