Gwen's Skill Max Order: Q → E → W
Level 1: Gwen can't do anything level 1, so run her down when she gets into range because she loses all trades.
Level 2: Gwen W makes her untargetable if you're outside the mist, but that's okay. I like to bait her W, then for the next 5 seconds just blind her and sit outside her W until it goes on CD. When her W is down, you have a 17 second window to make her cry. She is very weak without her W. If you want to fight her, you must be inside her W, but this makes her very strong, so only do this if she's low. She stacks conq fast and does a lot of damge when fighting you inside.
Level 3: Do not dive Gwen unless her W is down. If her W is down it is free, but if it is not, try to pretend to dive, bait her W, then reengage after if you have an opportunity.
Level 6: Quinn's E cancels her first AA in her R, which is good because she needs to hit her AA on you or use her W to unlock the second cast.
The main goal is to bait her W, and if she uses it, blind her and then walk back, Her W lasts 5 seconds, and I typically just wait for it to expire before engaging on her.
If I blind her she can't even hit me, so even though she is untargetable, I am too with blind. Play really aggro, and play around her W CD. Do not dive if her W is up.