Joust

Triple Pterodactyl is a relatively easy achievement to get. The method I used to get this achievement was created by a user on x360a.org called Grizimus, so credit goes to him for this guide. I'm merely spreading the word.

This can be done in almost any wave but its better to just go for this achievement in the earlier waves. Play the game as normal but when there is only one egg left, do not pick it up. Instead, leave it sit there and land on the middle-right platform on the higher part (where the gray spawn point is). Face right.
The egg should turn into a gray, which you can defeat and leave the egg again or dodge it (turning it into an egg again is easier). Now, if you've done everything correctly, the Ptero should spawn in the lower left corner and move its way right and up. Then, once above the middle platform, it will start heading left.
When it starts moving left, run to the right. If done correctly you should kill the Ptero. Repeat this method two more times and you'll get your 25 gamerscore.

I also have a video for it. The quality isn't the greatest but you can see what I'm talking about.

With the re-release of this game I thought I'd make a new guide since the person who wrote this one hasn't been on the site since January and posting a comment about this may not catch the attention of all people.

While easiest on wave 8, this and the one 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.