In Crysis 2, you need to collect Nano Catalyst from the alien creatures to upgrade your suit with different modules. These modules allows you to do different things like increase the duration of Stealth or deflect bullets when using Armor. Here's how to collect 5,000 Nano Catalyst in under 4 minutes.
You should have at least reached the 8th chapter of the game, Seat of Power, before you'll be able to do this trick. After watching Prophet's flashback and Hargreave's introduction, head straight to the 1st checkpoint.
After reaching the 1st checkpoint, just pass through the evacuation site and proceed to the building pictured above. Once inside, you should be able to detect a Ceph Guardian near the stairs with your visor. The Ceph will move to the highest floor as you go up the stairs.
Avoid stepping on the marked area above, as stepping on it will make the Ceph Guardian run away. Preventing you from getting that 5,000 Nano Catalyst upstairs.
As you reach the highest floor, avoid stepping on another marked area pictured above. Stepping on it will cause the Ceph Guardian to vanish into thin air without leaving a trace. It'll just literally disappear. Use your visor and mark the invisible enemy so you could somehow see it.
Since you are not allowed to move to the Ceph Guardian's location, just toss a grenade beside it to lure it out from hiding. It should run towards you while the grenade explodes in the background.
Wait for it to run outside the marked area and then fire to prevent it from running away. The Ceph Guardian is programmed to escape but it only has a limited time to do the action, so if you consume the time by attacking, then it'll stop moving afterwards since it's not programmed to attack.
After the enemy stops moving, activate stealth and then go behind it to do a stealth kill. Get the 5,000 Nano Catalyst and then run to the next checkpoint.
Once you reach the 2nd checkpoint, your game will auto-save. This will allow you to keep your current Nano Catalyst even if you load a previous save. Check your total Nano Catalyst by opening the Nano Suit menu to compare them later. Now quit the game and return to the main menu..
Select Campaign and then Load. Select your previous save data, the one before you killed the Ceph Guardian. Now you're back but your Nano Catalyst should have carried over. Just keep killing the Ceph Guardian until you are able to buy all the upgrade modules.
The Ceph Guardians are the final boss of this game. Not only are they invisible but they are tough as well. I guess the developers added one in this stage to show the player that the Ceph can use stealth. They probably didn't intend for you to beat one this early in the game, especially one that doesn't fight back.