When you go into the lair, you'll be gathering spores for the antidote to the titan. When you finish doing that and start to exit, around 110m from the end you'll be at a point where croc is constantly chasing you.
Remember when you laid that gel at the beginning?
When you're at the door, have croc come to the end and blow the gel when he's right on it. He'll fall down, and a cutscene will ensue.
50G will be yours!
Story related and cannot be missed.