I got wiped off the court the other week against a solid, flat hitter, they can be tough to face. I'd won every set against him in the past but this was one of those matches where I just had nothing in the tank from playing so much tennis beforehand (yep, tiredness excuse
I do like playing flat hitters though, and the key to beating them is to be well up for the match and be prepared to chase absolutely everything down. With such high risk tennis they will eventually miss and you can demoralise them by, for example, chasing down their forehand bomb and responding with a low slice to their feet.
Another way of taking them down is to be aggressive yourself. You don't have to stand toe to toe with them necessarily, but use some nice angles and simply don't give them the opportunity to wind up for the big bombs. The thing is with flat hitters, they are so used to being on the offense and usually have gaping holes in their games defensively so if you can find a way to take it to them - especially on your service games, it will pay dividends. A low slice is a great tool to use against them, and if you're going to use heavy topspin, make sure it's either deep (beyond the service line at least) or short and angled.