If you are looking to gain a lot of xp in a short amount of time, I would say that super paintball is the best game to do so. This is a fast game that can give you up to 400 xp if you get enough kills. Most games are less than a minute long as well. So if you compare that to a game such as bridges or survival games, you will find that you get a larger amount of xp in a shorter amount of time. I personally grinded this game to get from level 70-80 and I got about one million xp in a week. However, that is really hard to do, and you will need a good team to help you.