For this achievement you will want to load up a custom multiplayer match on Halo 2: Anniversary on the map Remnant. NOT Remnant Anniversary. I found the game type Sniper Slayer worked great for me.

Anyways load up the game and make your way to this area near the tower.

From there look to the left towards the cliff shoreline.

Look over the ledge and you should see some rocks in the water as shown.

Zoom in with your sniper and you should spot the BLAST Soda Can, shoot it and unlock the achievement!