I find flat hitting counter style players are much stronger on WT than other types of players, except for powerful flat-hitting power baseliners. However the way I beat them is to make use of the slice, its about the only time the shot is particularly useful. Counters get relatively weak accelerations off slices and short slices, since their the speed of their accelerations are determined by the speed of the incoming shot. I know slicing a lot isn't a very Serena-like thing to do but it does help
. Another thing I would suggest is to make use of short accelerations during rallies, drag the counterpuncher out of position and go for angled winners, maybe finishing off at the net if necessary. You kind of have to be careful though as they still can hit really good shots when stretched out wide.
About the return of serve, there's not much you can do except maybe step backwards just after the serve to give yourself a little more time to read the return. You're kinda gonna have to accept that counters hit great returns fairly easily. I think Manutoo is looking at CPU returns for the next build, so they might be a little more easy to deal with.