I did this by mistake, I was still going to the reactor and hiding for about 27-30,000 after 30 minutes............. that's the wrong way to boost.
You need to make an objective game and make it friends and invite only.
Switch teams to Resistance if you don't start as that team
Select an Operative Class
I always went after the right vent(probably can go left if you want) and it works much better with a character lower tier 2 or under.(level 13 and down)
What you do is you run in and hack the vent, when the AI comes (usually from the top middle the first time) you kill him.
If you stand right behind the hack box with the vent blocking your view from the door, the AI will always come in the room from that side and start to defuse if he doesn't see you. He will lock into the defuse even if he sees you after he starts (happens at level 20 too but the AI can stop defusing after a 1 minute or so if he is looking right at you). kill him and start hacking again.
Do this until the hacker hits 70-75%. After that you continue to hide on the left side of the vent right next to the hack box but about 3 steps back, with your characters body completely hiding from the site of the bot but don't kill him when he locks in.
Again seeing no enemy, the bot will run up to the vent and hack. Even if he sees you after starting he won't stop hacking (at lower levels), but to be safe you should hide to the opposite side of where the bot comes to defuse at. If he is to the left, you're to the right of the vent. And if he is in the middle just hide straight behind the Vent. Basically keep the vent between the you and the AI.
Now this is the important part, you start to back away from the Hacking vent until the percentage in the right goes from yellow to orange/brown. This means the AI is defusing more than you are hacking. It happens around 4 bars of connection or when you see your hack points are at +12 or +13.(At higher levels the AI could have engineer skills that let him defuse faster, so tier 3 players might have to sit closer)
Once you are in a sweet spot of inching forward and backwards in this range, make sure you leave it just in the orange/brown range. This will likely freeze the counter or worse case it will take the bot hours to defuse from 95%. After that you just walk away, go to sleep, change your car oil......what ever.
Because if you get this right the game will go into Overtime and keep going, after 120 minutes you should have about 170,000 exp if you stop hacking and let the bot defuse. Never Quit.
These are all projections because I was done with the game after 1 hour.
If you left for 2 hours it you should have 240,000 and a 15% bonus 3600.
3 hours would be 360,000 bonus 54,000.
4 hours would be 480,000 bonus 72,000.
So in 3 1/2 to 4 hours, you can go from level 1 to 24.
Sidenote: for level 13 and higher tier 3 plus players, the computer will be more aggressive because he has more skill/skills. The engineer will be much more likely to stop his hack to look for you if you are too close or to put down a turret or mine.
Level 20 players should check after 30 minutes to a hour to see if they are dead on the ground. You can get it to where he will leave you alone depending on the AI they set for that match, but things can go wrong with the super aggressive Bot.
He usually has the ability to shoot after you down him. Watch out for that bot if you are level 20 or higher.
*******Update:To avoid the aggressive Bot- " Start up Brink offline. This disables the DLC which is important. I can verify from testing that the so-called "smart" or aggressive AI only appears when the DLC is active. In the menus, now sign back into XBL and set up the vs. match, but this is the great part - the DLC will remain disabled."
The Level caps at 20 and exp at 300,00 but it saves all exp earned and it will unlock the next time you log in to the DLC
The Video pretty much explains it all, but with 2 players.