If you get tired of playing the same exact level over and over and over. Put the Superstar Ability on, and play the AI Battle Mode under the Training Section.
The following gives me on average 20,000 in 4 minutes. If I decide to make the games longer and kill more, I get over 28,000 points in about 10 minutes.
Make sure you also have the Injection Gun II equipped. It spawns in the middle, on the lowest level, right next to the structure that has the Plasma Cannon on it. If you set it to the "gold" mode instead of the "green" mode, and you shoot the ground around yourself, you will have no depletion of thermal when you shoot the Plasma Cannon for about 2 minutes straight.
Superstar Ability gives you 3x points in training mode. Play any of the battles, I prefer Team Elimination. Equip the Shuriken grenades (special disk grenades), Buster grenades (special normal grenades), and a Plasma Cannon.
Typically I play on the 1st battle arena since it is the smallest and fairly open. The Plasma Cannon spawns on the middle of the highest platform. If you're the blue team, this is to your right, and to your left if you are red team.
*Note* - When using the plasma cannons, make sure to pick up any thermal lying around, you'll need it if you want to get a lot of kills.
Using these weapons, you can basically sit next to a single data post, and dominate the terrible AI opponents.
If you get around 30 kills per game (takes between 5 and 15 minutes depending on how good you are), you will get about 30K.
The key here is to kill but not be killed. Getting lots of kills helps either way, but you'll get more points if you don't die often. There are a few AIs who are pretty good, but you shouldn't have to worry about them too much. The plasma cannon is your best friend, but make sure you keep your thermal levels up.