This one is easy enough - you simply have to fire paintballs at the green diamond markers.
The locations are below:-
1) Right from the start you can see it above a metal door.
2) Move to the first area. When you are on the second platform that drops down after firing at it jump off to the left onto a small ledge instead of right to carry on. From here jump to the top and then over to the right hand side. The diamond is at the end of the path.
3) Head back down the path you are meant to go for the mission. Go past the three platforms and take out the three artillery vehicles. You will see the third diamond in between the two guns that drop down and fire.
4) Move up past the rising platforms and through the electrified pinwheel. Go over the three platforms and up the platform you need to flip over. When you get up to the top via the two platforms the normal path goes to the left with the catapult. Instead jump into the ledge behind and left of the checkpoint. To do this jump from the little ledge onto the left and then backwards and make your way up. The diamond is above you.
5) Head back to the catapult but instead of letting it fire you up to the top, jump on so that it fires you across horizontally. This will take you to a platform with the fifth diamond.
The trophy unlocks straight away - you don't need to finish the level.