If you're unable to connect online, you can still play a training mission that tasks you with shooting dummy targets within a set period of time.
Experience is awarded based on the number of kills made for your team and the awards earned during a match. You won't always acquire the same amount of experience as kills made, however.
Along with Team Deathmatch, you can send Challenges to players on your Buddy List. These occur in real-time, though, and so the player you challenge must be online at the same time.
Committing suicide (i.e. firing a rocket at your feet or running into your own grenade) or executing a teammate subtracts a point from your score.