I got this on wave 12 which coincedentally was the same game I got the wave 10 achievement on. Of course, getting to wave 12 isn't exactly simple, all I can say is that I did manage to get into the 20s once on my Atari 800, many many moons ago.
I've discovered that it is far easier to dismount the enemy when you are facing the same direction as them, not to mention that the egg goes in a much more gentle arc and you're far more likely to pick up points as you score more for collecting eggs before they hit any of the platforms. Facing off against the enemy just seems to make them flap higher than you rather quickly.
Oh yes, dont hold the control as normal - you need to use your index finger on the A button and not your thumb - it's the only way to tap at warp factor 9 and Joust is an old fashione Daley Thompson's decathlon keyboard masher in this respect.