how do i beat garen top as riven?

Thats all you do with riven anyways so thats not helping obviously...

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Early game, learn to proc your power bonus auto attacks... for instance, if he's gonna Q you, you'll be silenced, which means, you might as well trade if you can -- so, before he Qs have some Q's of your own done, to proc your bonus AD -- you can kite him with some hard auto attacks, in fact, i try to land my third Q on him when he goes to silence me... When you both get a second ability, if he goes for his spin, thats when you're going to want to shield away -- if you've been silenced and he activates it, well you've already screwed up... but that's okay... level 3 - you'll have your stun if you didn't take it already, when he goes to Q you, stun him as he gets in close - tricky to do but its what will make the difference... Then, you just gotta play it smart, when he spins, you either choose to shield away or stun him to stop him, you dont want to take too much of that attack, in fact avoid altogether... level 6 time, well if you're going to engage him, always have your ult and a fair bit of health, the trick to Garen is you never want to be too low vs him, get in the habit of taking TP vs this matchup... I beat Garens all the time, you gotta be careful not to let him get ahead. Using the tactics I've stated you whither his health down, when he's at about 50% you can engage with ult and kill him, prodvided you have about 75-80% health. Anyways... it takes precision with Riven's abilities to beat a Garen, or just have your jungler camp you hard if you are having difficulty.

I'll add another little tactic -- when you're engaging him, after 6 - for the kill... when he goes to silence you, shield dash away, he will follow up with a spin, but you've already gotten distance with your e vs his Q -- then, set your Q up, auto attacking between each one (you want to wait for his spin to be nearly done) then give him some damage -- then, stun him if he tries to run, repeat a shield dash with auto - get your Q's up if needed, release windslash, be prepared for a flash (if he has it, aim accordingly to where you think he might flash, while still having his current position in the path -- its tricky, and a good player will probably outflash despite your readiness for it, but you're losing to a Garen, so you're probably not super high elo -- so I doubt he will have amazing flashes himself)... that's about it... its rough and un polished in terms of advice, but really you just gotta learn Riven and how to do this -- once you figure her out you learn what tricks/abilities to use at what time and the precision involved.

but by then hes going to start spinnning as well, he always wins trades with how tanky his w makes him.

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You beat him once laning phase is over.

he had warmogs sunfire and randuins, he just focused me in teamfights and i couldnt get skills off. if it wasnt me, it was my adc being focused. his silence sort of destroys in teamfights.

This is probably the best answer here. A good can out duel an inferior , but assuming equal skill this is an extremely negative match up for Riven. You wont be able to kill him if he knows what he is doing, but you can outfarm and out roam him which is what you want to try and do. The main thing is to farm then when he comes in for teh silence stun him with W then E backwards, if he spins stay out of range and counter with broken wings when it ends. If he doesn't spin, go back to farming until he goes for the silence again.

Yes please build MR against garen, it will make garen's life so much easier. But seriously if you think his abilities do magic damage maybe you should take a closer look against who you're fighting. The only magic damage he does is when he gets sunfire, but mostly his damage is AD.

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Pick a different champion.

he counter picked me by getting garen.