Like GlennMirnyi I also enjoy playing against flat hitters, because as he says, you can use their pace against them. Saying that though, one of my friends at my tennis club made mincemeat of me in a match last year with his flat forehand, the conditions were quite damp and the ball was skidding through at an awesome rate - it was one of those days where I simply couldn't get my racquet to the ball to do anything, so I can undestand the difficulties you're having.
I think you've had great advice from everyone so far, I'd just add that early prep is vital on your shots, get your racquet back early and anticipate a skidding, penetrating bounce - you'll need to start your swing slightly earlier than usual. Do you have any friends who play like this? It might be useful to have a few practice sessions with them. I play in a league with players of all ages so I'm pretty much used to all the different styles out there. Just see it as a fun challenge to overcome