This Serious Sam 4 Guide and Walkthrough will help you through the mission Man vs Beast.
Lead The Attack
Watch the cutscene where you are being introduced into the characters and what they wanted to do and the main protagonist, Serious Sam.
Right after that, the camera zooms into the planet where you will find yourself on the battlefield fighting thousands of enemies. You have all kind of weapons so feel free to try them out while you can.
After a few seconds of fighting and killing all the bad guys, a huge guy appears and knocks you down. Now you have to watch the short cutscene where here beats you up! This very short intro level ends here.