You're beginning the stage in the narrow tunnel. Leave it by destroying the wooden blockade.
You'll find some ammunition outside, so take it. Do the same with the spear lying close by - it'll be useful thereafter.
Before going up the first stairs you should see plenty of monsters flying in the distance.
Use sniper's sight and start enemies' extermination. Some of them will hang on the stone building - eliminate them with ease. Shoot down as many flying monsters as you can, but save at least two bullets for unexpected troubles.
Don't shoot opponents feeding on the ground - use spears and bones to do that.
If you haven't killed enemies hanging under the ceiling, then be carefully during the walking up the stairs and be prepared to defend yourself.
Now, enter the huge cave and move ahead through the arch (there won't be any enemies to fight off).
Turn right and go through flooded tunnel. You'll reach the place, where Kong is sleeping and where Ann is. Call her and then sail away. Unfortunately, some never seen water snake will show up. Ann will scream and wake up Kong.