This Glitch will make the rest of the game pretty simple and you will get all the other trophies or achievements with minimum effort.
To perform this trick you need to find a side quest that has no cutscene in the end! Do what you need to do, go to the location where the quest will end and use your weapon. While putting the weapon back you need to talk to the contact person. Now you just need to press the same button over and over again and your level will rank up faster than you can watch. Side quests can be found throughout the whole game so it is really simple to get this done.
At the moment this glitch works on all consoles!