I know he needs buffs but let me give you a little tip in the mean time, I main this guy and I did a lot of experimenting with him for a long time.
He has scaling armor and unlike most champions he also has scaling MR for pretty much the exact amount you got on scaling MR glyphs.
So just get full scaling HP per level glyphs, seals and quints and a dorans blade to start, it REALLY helps his early game and lets your damage items you need and takes that feeling of squishiness away from him.
I'd get attack speed marks also.
Start with a blood thirster (fuck hydra's) and you'll be able to duel riven no problem every time ur ults up.
As long as you do all this, at level 6 at least you'll be gaurentee'd to be able to duel her, though I've honestly beat her earlier at level 2-3 and such.
This isn't mandatory but I'd lastly suggest getting summoner heal, its a great spell for him, when dueling if you feel you arn't going to make that third heal win your duel u can use it AND get that next heal, it pretty much always wins me duels early.
Late its good for when u jump in and kick some ass but then the next person has the ability to kite you, it lets you either chase while getting some sustain regardless of attacking or escape, it also helps your team in team fights.