I laughed.It's pretty much like playing vs a "Katarina" from LoL.
If you feed her early she will use that for snowballin, it's simple..
Don't die, be cautious and gain vision on the map to be prepared for Flash..
Flash can't do anything, if you stop him from doing anything, it's so simple.. Yet.. The lack of teamwork disrupts ways to take down the Flash, since you need burst and crowd control for him, but it will be worth it, Everytime!
Underfed Flash has a really hard time making a comeback, unless your squishies go alone and Flash takes them out and makes a comeback..
But that's your fault then for not predicting that..