Hey Alex, I know the problem well, a lot of players I work with find this serve difficult to return. The best way to return this serve is to bebmore brave and really try be positive and topspin firmly over the top of the ball fairly quickly. This makes the side spin take less effect the more of your own topspin you can add.
Just focus on hitting right over the top of the ball and with a nice short shot brush quickly through. Quite often plaers miss this serve as they are too careful and the side spin takes it long or off the side. Give that a try and let us know how you get on!
Dan
