This trick can get you from level 2 to 30 in an hour and is a lot of fun to do.
The only requirements are that you have not yet completed Operation:Anchorage, and make sure you use the sniper rifle for the final quest "operation anchorage". Also you'll want to bulk up on .308 ammo by doing the "ammo" cheat.
BEFORE ENTERING THE SIMULATION:
Before entering the simulation try to get at least 50 in sneak and concentrate on getting this skill as high as you can by the final quest of the sim. This will make things a lot easier on you. By choosing both explosives and sneak as 2 of your 3 tag skills you can also disarm the megaton bomb for a quick level up and add all the skill points in sneak.
GETTING YOURSELF PREPARED:
1-Save your game BEFORE speaking to the quatermaster the VERY FIRST time. You'll want to pass the speech check to get the GAUSS rifle. This will make unarming jingwei much easier. Unarming him is important since he doesn't behave the same way unarmed as he would with his sword (making it easier to stay hidden from him later on). Plus if you did the "GARY" trick you get a pretty cool unbreakable version of the sword (not to mention the gauss itself). Since this speech check can be difficult to pass (especially at low levels) it's best to save and re-load should you fail.
2-Since you can only do the "XP" trick at the very end of the simulation where you meet and defeat gen. jinwei, then you can go about the simulation as you normally would BUT make SURE you choose the sniper requisition equipment for AT LEAST the final quest "operation anchorage". After speaking with the quatermaster and receiving your sniper rifle go inside the tent and bulk up on .308 ammo. To do so press on the ammo dispenser to fill your ammo (if not already full) then go into your inventory and drop the .308 ammo and then refill your ammo again. Repeat this until you have AT LEAST 1000 bullets (you'll need them) best to make it at least 2000 just to make sure. The reason the sniper rifle is needed is to speed up the process by getting 1-hit headshot kills. The combat shotgun works well if you already have at least 70 in small guns.
3-Now you got the GAUSS and you have your sniper rifle with at least 2000 bullets, there's only one last thing. Do NOT speak with sgt. montgomery at this point. The reason for this is that ONLY you and benji can actually hurt and kill jingwei so if you bring him along he'll kill jingwei a mere minutes, ruining your chance of doing this trick. So do not even speak to him, leave him behind. Of course this means you'll have to do the whole "pulse field" section of the sim by yourself.
If you followed the steps above then you are ready to go. Go through the entire mission as you normally would (except w/o benji). Once you reach the switch that turns off the pulse field the power armored soldiers will blast their way to the chinese HQ. Before entering the door save your game. The reason for this is twofold. If you do not choose the third speech option "i'm so going to kill you" when speaking to jingwei, you could talk him into surrendering thus ending the sim right there w/o the chance to do the trick. 2nd reason is shooting his sword out of his hand can sometimes be troublesome and you can take a considerable ammount of damage in the process. So if that happens you can simply re-load.
When you first enter the compound (HQ) you'll see jingwei execute a soldier then you'll speak to him. Notice the large metal crates at the far right-hand side. This will be your destination and vantage point to do this trick. Not only will it offer a close range shot but also it shields you from american soldiers' fire(which for some reason are the only ones besides jingwei to attack you). It is also the only place you can hide from jingwei.
Now tell jingwei you'll enjoy killing him and shoot his sword out of his hand. Make sure to pick it up quickly. If you shot it out of his hand with the gauss rifle, jingwei might have time to pick it up before you are done reloading the gun (although uncommon still frustrating). Now that you have picked up the sword, jingwei will still chase you but not as relentlessly as he would with his sword (he'll run in circles instead).
Now run to the aforementionned area, jump on the barrel near the crates and then jump on the crates. You can't access the higher crates but that's good as they shield you from enemy fire. Watch out not to fall behind the crates you'll become stuck and you'll need to reload. Hopefully at this point you have at least 70 in sneak, this will make it a lot easier to remain hidden long enough to level up. As you know you can't level up when you are under "caution" or "danger" a high sneak will make it easier for jingwei to lose sight of you long enough to level up. However, even if you don't level up the XP stacks, meaning once you are out of the sim you'll gain maybe 20 straight levels if you have enough XP earned. That in itself has only one negative side effect, if you did the "GARY" trick to recover the sim items into the "real" world the window of opportunity will be considerably shortened as the "level up" screen will appear as soon as the sim ends before you return to the outpost.
Back to the trick.....Once on the crates you'll notice a small door in which chinese troops come out of. For every chinese soldier that is killed at this point will immedietely be replaced and will come out of that door. So naturally aim your sniper right there and head shot them as they come out. Once they come out of the door the soldiers pause for about 2-3 seconds before running into combat that's more than enough time to headshot them.
If you can 1-hit kill them, then i strongly suggest raising the difficulty level as that will provide you with more XP for each kill, speeding up the process. A critical sneak headshot with better criticals (perk) and 100 in small guns will 1-hit kill soldiers on very hard. On normal difficulty from level 6-30 you'll need to kill about 900 soldiers. So if you have 2000 bullets you might not want to waste too much ammo on higher difficulties if you can't 1-shot them. Also bear in mind that you get the same amount of XP for assisted kills. So if you only wound a soldier let the american troops kill them off. If you went all out and have 3000+ bullets then feel free to go out of your way to kill them yourself. You'll notice that you'll level up EXTREMELY quickly and inside the hour you should have reached the level cap.
Try to save every 1-2 levels, eventhough this process is very quick something un-expected might happen like an explosion might knock you between the crates and the wall unable to move and forced to reload or just the game freezing (didn't happen to me but why take chances).
Sometimes, rarely even, even if you are "hidden" you might not level up. This sometimes happens and you need to enter a new zone to level up. To do so, go back where you entered the HQ eventhough there is a "blue force field" in your way you can do the "lucky shades" trick by entering 3rd person camera and moving the camera around to exit the HQ (although if you are only one level past you last saved game reloading might be quicker). This will probably not solve the problem right away since jingwei will follow you. You need to re-enter the HQ and return on top of those same crates and return to "hidden" status, this will jog the game and allow you to level up accordingly.
If you have below 70 in sneak you might have a hard time hiding from jingwei. A good trick is to go as near the edge of the crate right before falling and getting stuck between the crate and the wall and of course remain croutched. This should make you unseen by jingwei and allow you to level up.
Be warned that if you exit the sim at level 30 that albino scorpions, deathclaws, ghoul reavers and mutant overlords will be VERY frequent and if you entered the sim at a low level you'll be low on ammo and weapons to deal with them. So make sure you loot the armory for all its worth. The gauss will be especially useful until you stock up on typical weapons for a level 30 player.