With the re-release of this game I thought I'd make a new guide since the person who wrote the last one doesn't seem to be registered on the site anymore.
While easiest on wave 8, this and the triple Pterodactyl can be done before that, you get at least two on wave one, if you are willing to be up for the challenge. To kill a pterodactyl you have to stab him in the mouth with the lance. They seem to appear around once per minute give or take a few seconds (while going for the camper achievement to survive 2 minutes in wave one without dying) 2 appeared.