Some good (ie., testable) comments so far, imo. I'm not qualified to offer any advice, but here's some vids (they're all really short, like a few seconds each, so go ahead and check them out).
This one's hit at about chest height. I guess I could have taken the ball a little earlier (a bit higher). Anyway I think the principle is basically the same as for undercutting higher balls.
These are hit at about shoulder height.
This one's over my head at contact. I followed it in because I figured I had enough time to get into a fairly good net position. Probably a bad idea on such a no-pace ball, but this is 3.0 tennis where nobody has good passing shots and any kind of pressure usually leads to errors, and anyway I should have been able to hit a successful volley off what came back.