There are 15 multiplayer missions to get through. They are as follows:
1.) The Junks Are Back in Town!
2.) Billy the Bin
3.) Four to the Lava
4.) Serial Lava
5.) In a Wormy Submarine
6.) Give me Five!
7.) V for Vendetta
8.) Space Wormity
9.) Saint Pete's Apostles
10.) Space Dividers
11.) Food Intoxication
12.) Gastric Issues
13.) Spikes Peak
14.) Chips Chop Back!
15.) Bump a Dump a Diddle Doo
You simply have to complete these levels for this achievement.
If you're having trouble getting through certain sections if this is your first time playing the game, remember that certain enemies can only be killed in certain ways and it may be difficult to find out how to progress at times:
a.) Some enemies can be stunned or suppressed with your whip, allowing you to jump over and get by them.
b.) The plasma guns (look like machinegun pick-ups but they have red energy swirling around them) are very useful and can kill certain enemies that you normally cannot kill with your machinegun.
c.) Certain parts of the level require you to look for a hook to use your whip to get through the area. They may be difficult to spot at times.
d.) There are certain spots where you simple have to push the controls to the left or right to go up or down to get past a slope, where normally you would think you be able to jump over.
e.) Using teamwork to step on or whip switches helps!