As you can see from the time limit above, this is a very short mission. When you get to the marker on your map, just run into the Colossi units on the beach to officially start the mission. This is also one of the easiest missions to get an S rank in, as long as you make sure you defeat every single enemy and harvest their Phantomas.
To begin with, you will have Imperial Troopers and Supersoldiers to deal with, as well as a Mid-Sized Carrier. If you’re not quick enough, then the Carrier will fly off and you won’t be able to get the S rank (unless you’re solo). After they are all defeated, two Colossi (named Campini Squad) will come out of the water, so focus on one at a time and when they’re both finished, the mission will be over.