When you enter the lab with the mutants, shoot and kill the scientists there and cause chaos. After this you can hear one of them say "I'm not even suppose to be here today!" referring to Dante Hicks from Clerks.
In the levels after Riddick leaves in his submarine and the base is on self destruct, you will eventually reach a point where you crossed through water before. Go back into the water and find the point where you first dropped in. There is no ladder to climb back out. There is a secret box here. Swim to the upper right corner in the water and you will see a cross eyed fish.
Soldiers are a very tough enemy to fight at first. An easy way to defeat them is to get very close with the shotgun, usually touching them. However, if done on a long strip of land with no cover, you will not survive long. Try to hit them in the head with the pistol, or spray just above their head with the rifle while zoomed.
Use the following trick to defeat the large monster in the wooden area. Drop to the right immediately after you go into the pit. Let him charge at you until he is in the corner. Stun him with the pistol, then run back to where you started. There should be a flammable barrel nearby. Shoot it and try to catch him on fire. This deals great damage, and you should only need several more pistol shots and a couple shotgun blasts to finish him off.
If an enemy throws grenades at you, you can throw them back by pressing E. However, grenades have a small blast radius. You may not be in as much danger as you think.
At any vending machine, press any button rapidly. A health canister will drop out. This can be done twice. However after this, a live grenade will come out and endanger you, unless you throw it back or run away