From Halopedia, the Halo wiki
Laser Blaster is an Achievement in Halo 3: ODST. It is awarded for getting 10 kills with a Spartan Laser during the ONI Alpha Site Campaign level. It is worth 5 Gamerscore and is quite easy to do because players start off with a Spartan Laser and later in the level there are two others: one on the second level of the stairs before the ONI building, in the central structure (the one without the turret), and the other just around the corner to the left at the top of the stairs, facing the ONI building.
The achievement can be done at the beginning of the level as there are 3 Wraiths that rewards the player 3 kills each, requiring the player to receive only one kill with the last two shots. It is also easily obtained by looking straight from the sandbags at the end of the bridge before detonating the explosive charges and firing shots right across. A single shot can net up to 5 kills if aimed properly.
It is represented by a red and silver signpost with a Spartan Laser in front of it.