PAIN

Make sure you do this in the Fun With Explosives environment (not Freeplay). They are all fairly easy to hit. You don't have to hit them all in one run and you can reset the scene between each. Just make sure your character actually hits them. After you get the 12th the trophy will pop.

Order of presents in the video below, going clockwise from the launcher:

0:00 In gondola transfer area
0:12 Small platform left of slalom ramp
0:20 On roof of building to right of slalom ramp
0:32 Near ski lift transfer area, behind snowboard rack
0:49 Behind warning sign, to right of train tunnel
1:05 On platform behind scaffolding, above train tunnel
1:19 In Unabomber area, use tunnel to the left of the ski lodges to reach
1:48 In front of door on left ski lodge
2:01 Roof of left ski lodge, use geyser to reach
2:22 By fireplace, beyond the hot tub
2:34 Roof of right ski lodge, use geyser to reach
2:51 Exit of second tunnel, use the entrance by the geyser to reach