To unlock this achievement , you will need to find the secret exit of level 11 (Dino might!).
The exit will be in the bonus path, to which we can access after taking the yellow gem, obtainable by successfully completing the 7th level Death Route (Hang 'Em High), accessible only through the secret Warp Room.
After taking the yellow platform in the Dino Might! level, which will take us to the bonus path, we will not have to do anything else but cross the whole level until we reach the triceratops. At that point, we can break a Checkpoint box and go on.
Ok, now you go into action.
We will have to be captured by the second pterodactyl that we will find when the triceratops will start to chase us.
The pterodactyl will not make us lose a life, but it will take us to the Eggipus Rex bonus level.
At this point, the achievement will unlock itself automatically.
BONUS WARP ROOM SECRET, HOW TO UNLOCK IT: simply getting 5 relics. After doing this, we will unlock a platform, in the main Warp Room, which will take us to an underground area, where there will be 5 more level.