What is the fastest way to level an account?

The fastest way would be Playing COOP VS AI-games on intermediate level until you reach level 14 (You gain normal experience until then). At level 15 the exp gain will be lowered from bot games if you've already played more than 180 minutes of bot-games per day, so you have to play normals. There are 2 options:

• If you know that your 5v5 games last rather long, play 3v3 since these are usually much shorter.
• If you know that you easily win games fast always play 5v5 since these give alot more experience if they don't last too long. After 40 minutes it's not worth though. Two 3v3 games that last less than 20 minutes will give you more experience than a 40minute 5v5 game.

Also remember to buy EXP boosts. If you win more than 4 games per day you should take Timed boosts over Per-Win boosts.