I tried playing Riven against Garen. It was kinda painful. I only died once but I was not able to do the pro stuff and outtrade him. He got 280 armor or something with thornmail and cape of suns and other items, and he started two dorans blades.
I started one dorans blade but maybe armor and 5 potions would have been better. I'm not sure.
I tried ultimate combo power from that cool youtube video, but I think I would typically do the EW stuff anyway, so I'm not totally sure I'm getting all the differences. I also tried the AOE tiamat evolution item thing and it worked pretty well on minions.
I ended up finally passing him in farm after he was beating me in farm, and though he took the top tower, I only died once and ended up with fewer deaths than him, and the same kills and assists if I recall correctly. But, his team ended up with a super powerful scary tank and my team ended up with a scared squishy Riven. I always fear building tank items on Riven is a recipe for disaster and you've got to be damage-y and hope you can time the 2 second shield to use the AD scaling properly.
We fortunately won since I think the Garen did not go back to heal and tried to save the second mid tower, resulting in all of the other towers and the nexus going down. =D