Each time you complete an online event you will earn XP towards leveling up. Your total XP will depend on the following four factors:
Number of people in the game (the more people, the higher your multiplier will be)
Trick bonus (the more ticks you pull off, the higher your multiplier. Caps at 1.5)
You will need to earn a total of 260,000 XP to reach level 25. Obviously the better you perform, the more people you have in each game and the higher you get your trick multiplier, the faster you will rank up.
Click HERE to find people to boost this with.
TIP: If you are working through all the other MP achievements with other people, you will probably be around lvl 22, from here just play games of Infected until you hit 25.
Infected offers the most XP per time spent playing as these games can be over in a matter of minutes and as long as you place in the top 3 you can earn up to 6500 XP per game.