Tomb Raider 4: The Last Revelation

To kill skeletons:
The skeletons are invulnerable from any of Lara's weapons, but they can be killed. To kill them you need use a shotgun. When you use this on a skeleton they will fall backwards. Do this near a big drop and watch them fall down to their deaths!

Shortcut to beat Van Croft:
The whole of the second level is a simple race against Van Croft. To beat him here are a few tips:

When you need to swing on the bars do a running jump to get a headstart
When you need to jump over two pits, fall down the second pit and head for the right hand corner and there is a shortcut so you can get in front of him!

Level Skip:
Go to the inventory screen and make sure that the compass is facing north. To get the compass facing exactly north, hang from a ledge facing north. Otherwise, the cheat won't really work. While in the inventory screen, go to Load Game, press and hold H + E + L + P then release those keys and exit the inventory screen.

All Items:
Go to the inventory screen and make sure that the compass is facing north. While in the inventory screen, go to small Medipack, press and hold G + U + N + S then release and exit the inventory screen

All Weapons, Infinite Ammo, Sml Medipack, Lg Medipack:
Put in the All Items code then go to the inventory screen and make sure that the compass is facing north. While in the inventory screen, go to Large Medipack, press and hold W + E + A + P + O + N + S and exit the inventory screen

Harder Route to the Iris:
The whole of the second level is a race against Van Croy. If you are a true tombraider then you will be interested to know that there is another, alternative route!
To access it you must first collect all 8 of the golden skulls on the first training level.
This route is much harder though!

Gold Skulls in the Training Level:
--The first skull is at the beginning toward the left.
-- The second skull found is in the shallow water below the very first jump you are instructed to do.
--The third is below the running jump section of training.
--The fourth one found is in the water of your swim training session.
--The fifth one is found during the outdoor sequence where the old man is fighting two of the wild boars. It's around to the right, all the way into a right hand corner.
--The sixth one is on the floor on the right side of the room where the spikes are near the exit door just before Lara goes out to learn to crawl through the crawlspace.
--The seventh one is the one in the crawlspace (you called it the fifth one.)
--The eighth one is on an elevated ledge/walkway that's off to the right where Lara follows the old man up the ramp just before he makes the speech about going left or right depending, because going left is harder.