For Both Of The Cheats Below, You Must Be Facing NORTH. To Check If You Are Facing North, Open Your Inventory. The Red Compas Nedle Will Be Transparent If You Are North*.
All Weapons: Highlight Small Medi Pack in your inventory. Press G+U+N+S together. It may take a couple re-loads of the inventory to show, but you will get all weapons and some weapons will have unlimited ammo.
Skip Levels: Highlight Load Game in your inventory. Press H+E+L+P together. Exit the Inventory. The Next Level will load.
*= An easy way to face north is to find a ledge and hang from it then a) climb back up or b)drop down again. Check Your Compass. Keep Going Until You Find A North-Facing Ledge!
The skeletons are invulnerable from any of Lara's weapons. They can be killed if you use the shotgun. When you shoot the skeletons, they will fall backwards. Shoot them when they are standing near a drop and they will fall to their dooms!