Just a warning, this achievement is not easy. It is insanely difficult.
The only thing I can recommend that the wave 30 and 100 guides can't offer is a coop strategy.
The best way to get to wave 20 in COOP is for each player to play only one side of the screen. Great communication is essential. In egg waves, the player with less lives or more deaths should take the majority of the eggs.
Or one player can take the top portion and one player can control the bottom portion of the screen.