Tron: Evolution

You level up by gaining experience, which is earned from defeating enemies, completing campaign objectives and finishing matches online. It is important to note that campaign difficulty does NOT affect XP earned so if you are going to grind for experience you may as well do it on easy.

The first time you complete the campaign you will be around level 20. At this point the fastest way to grind out most or all of the remaining levels is to keep replaying level 6 while still on easy. The tank section is the fastest way to gain experience in the game. Your experience saves once you get a checkpoint, so you can choose how much of the level you want to play. There is a checkpoint when you exit the tank, another one before the boss fight and one after the boss fight (the start of level 7). Play through whatever portions you want to, then just exit the level and replay it again. It will take approximately 4 hours to go from level 20 to level 50 by JUST replaying level 6.

Of course you will also gain more experience while playing through on Hard/Insane and any online games you complete.

To give a rough idea of how long it would take to get to level 50 by just playing through normally: You will gain level 10 just before chapter 3, level 20 before the first ending, level 30 before the "insane" ending, and the next 20 levels can be obtained from about 8+ hours of online play (experience gained online will vary depending on skill).