As an L7 Garen player (Though someone like Critmaster Garen could probably explain it better) I'll try to explain things.
Garen, like most other Juggernauts, has an Auto-Attack resetting skill, his Q. This makes it synergize very well with Auto-Attack procced Keystones like Grasp, or Conqueror and others.
In regards to his kill pressure, it simply means he is doing his job as a Juggernaut. Juggernaut champions are DESIGNED to be able to deal out large amounts of damage, while also being able to tank large amounts of damage. But this comes at a cost. The cost being among some of the slowest champions in the game, with very limited (Or in the case of
, none at all) Crowd Control.
While simple on paper, Garen's kit (Thanks to his Visual Update and the Juggernaut Update) DOES have its complexities.
Garen, unlike a Juggernaut like, say
, does NOT have a strong early game. His Villain passive on his ult, for example, ONLY comes into play once he's LVL 6 at the earliest, AND a member of the opposing team needs to have gotten at least 1 kill for it to even activate; while
gets his Noxian Might at LVL 1 as long as he can apply all 5 stacks of Haemorrhage. In addition to that, his lane opponent might not even BE the one that gets made the Villain depending on how the kills are flowing in a particular game. In low-kill games, for example, the Villain passive can be near-on useless because there might be a long period between kills, and thus, no one on the enemy team is getting marked as the Villain.
Garen's W's passive that gives him free armor and magic resist requires him to get good CS rates, as kills (Whether of minions or of champions) are the ONLY things that increase it. Thus preventing him from farming not only reduces the amount of gold he can get, and thus items he can buy, but ALSO keeps him from getting stacks of his W's passive, making him less capable of tanking damage. His W also has a LONG CD, meaning if you can get him to use it prematurely, you've taken away his defensive capabilities even more.
His E does 33% increased damage when hitting a single unit. By standing with another champion, or in groups of minions, you prevent him from getting that potential damage boost. It can also critically strike, but due to the way most crit items work, it's not exactly the best idea to build it on him. (Critmaster Garen or others might take flak with this, but just speaking from my own experience)
His normal passive, his Perseverance/Regeneration, can be put on cool-down with something as simple as an auto-attack. Damaging abilities can also put it on CD. DoT (Damage over Time) abilities are even MORE effective as each tick resets his passive's CD. Constant poke keeps his passive on CD, thus greatly reducing his ability to regenerate and sustain through the laning phase.
Do it enough, and you could (Depending on your chosen champion) get him low enough for an all-in, as by constantly poking him, you prevent him from regenerating.
Let's Continue. Garen, like all Juggernauts, is susceptible to being KITED. He's actually one of the MOST susceptible, because unlike someone like
or
or some other Juggernaut, his CC is not a Stun, Slow or Pull or something else. It is a Silence, which, yes, prevents the use of movement abilities such as Dashes, but it does NOT hinder actual movement speed, meaning you CAN move away from him after he's used it, and he largely can't do anything to stop you.
His play-making abilities revolve ENTIRELY around his
, because, like I said above, he doesn't have a Pull, or a Slow, or a Stun for his CC. Get him to blow his
and you've reduced his play-making abilities even more.
This is probably a way too long, and way too wordy response for such a short post as yours, but I'm making it anyway.