Watch the "Get the First Body-Armor" tutorial in the Video section to see how to fight the lake dwelling beast and acquire the first body-armor in the game. It also shows the next path in the map of where the ramps to the upper levels are from "Ground Zero".
NOTE: You will need ALL the body-armors in order to properly use the more powerful weapons in the upper levels & increase your health. Without armor, the blast from these weapons can kill you.
This video starts off from where "First 5 Minutes" left off.