Savanna

As I said, it will take time to eat away enough of the beetle's shell to get to the vulnerable bits, time being an especially scarce resource on any battlefield.

Feeling the unwanted passenger clinging to it's hide, the beetle retreats, scuttling back down a ways into it's hunting tunnels. Unless Red detaches very quickly or before the beetle can get underground at all, he'll soon find himself trapped between a rock and a hard place as the beetle begins slamming it's back into the walls of the tunnel, trying to crush Red or at the very least scrape him of it's shell.

The good news is, if he can survive being repeatedly slammed into the ground with immense strength, Red should have absorbed enough shell material by now to form a dagger or maybe some claws out of it!