You will fight the First Boss at the end of Level 4 - The Coast.
The Squid with a Huge Gatling will shoot you with the gatling. Never stay in front of it when it's shooting or you will die in seconds.
If you touch the boss with Grood, you will die instantly.
After a while the Squid will launch guided rocket at you. Use your slow motion to avoid them if cornered or try to pass between them, usually there is a gap through the rockets but you need to be quick.
When the Boss dies stay away from him as when it is falling to his death you can still die from touching it.