Let me start of by saying that I don’t really like the idea of leveling in shooters. For me the fun in Brink is in playing it and I would like to have everyone have the same lvl and abilities. Fortunately Brink was designed to get to lvl 20 relatively quickly.
But for those of you who just want their basic lvl 20 chars I am going to describe a method a friend of mine (Rando) discovered which works without you being in front of your PC at all. Everyone else who thinks that this is Bull**** and one shouldnt do so just stop reading.
I realise that this is probably going to get fixed in later patches beacuse it really is kindoff an exploit but by then I should have enough lvl 20 chars 
What you are aiming for is gaining 225k XP in just one Match. Why only 225k XP? Because if you manage to win the match you will get 15% completion bonus. Also if you play in ‘versus’ mode you will get the 20% online bonus. So you need to set up your listen server in versus mode but not let anyone connect to it which is really easy if you are behind a router and have not setup port forwarding. In the end you will have gained 300k XP and become lvl 20 in just one match!
This whole method is based on you being Resistance and hacking the last objective on the map Resort and the lonely bot (which is always going to be an engineer) dehacking it at the same time and taking that into overtime. So you need to configure your server to have one player and one bot in objective mode with overtime.
You want to get to the last objective on Resort very quickly. You also want the bot to start dehacking at around 90% progress to make it easier to win the map in overtime. It works really great if you hack it to 95% and then run away with the hacking tool such that there is no more real progress being made because the connection is low. Then you wanna wait until the bot starts dehacking. As soon as you see the progress getting lower you can walk into the room adjacent to the hacking box and just stand behind the wall which is 2 meters away form the hacking box. That way the hacking and dehacking speed are exactly the same and the progress doesnt change at all. Now you can change the game to window mode open the console, minimize the game window and do whatever you want. After roughly 2h and 30 mins you should have 225k XP and you need to start running in circles and distracting the bot so that he shoots at you and stops dehacking. All the while you cant stop hacking because your are in overtime. You need to watch out for the meele attack, once he knocks you down you loose. If you manage to win the round you will end up with over 300k XP.
There is one important thing however. The bot cannot have ‘Nerves of steel’, because if he has he can dehack a little faster than you can hack. It is however very unlikely that the bot has it when you start a char from scratch.