You just need 2 controllers for this. Start an Infiltration match on the first map with no time limit and however many wins you want.
Bring your first controller guy over by the other teams goal on the ground. Keep jumping and shooting which will go towards this
Then score a goal. Your mimic will come out and jump and shoot as you did.
Now with your second controller keep coming down to get killed
If you watch my video starting at 6:30 you can see me doing this.