The M6 spartan laser

The M6 Spartan Laser is a man-portable, shoulder-fired, directed energy weapon used by the UNSC. When the trigger is pulled, a red targeting Laser is painted on-target accompanied by an audible whine as the weapon cycles up. The Spartan Laser will then charge for approximately three seconds before discharging the main laser. As the Spartan Laser charges, the charging tone gets noticeably louder up until the laser is fired. The firing sequence can be aborted without discharging the battery as long as the weapon is not actually fired; for example, if the target moves out of sight or the operator needs to displace. The laser is capable of penetrating several enemies or vehicles, and has a small blast radius, injuring targets within two meters of the point of impact and toppling light vehicles. It has an offline sequence between shots while it (presumably) cools down (approx. 2-4 seconds), and is highly effective at destroying enemy vehicles.[2]
In Halo 3 Multiplayer, if you kill another player with the Spartan Laser, you will receive a medal, the Laser Kill Medal. It is likely awarded due to the difficulty of having to charge and aim the weapon. Also, the achievement "Two for One" is unlocked by scoring a double kill with a single laser shot in any ranked free-for-all playlist.
The weapon is not available in the campaign until the level The Covenant, which Master Chief begins the level armed with the Spartan Laser. There is another Spartan Laser lying nearby on the ground, presumably dropped by a Marine, but only if played on Easy or Normal. The M6 is the only weapon able to harm 343 Guilty Spark on the final level of Halo 3, with the exception of the Fuel Rod Gun and Rocket Launcher, through a glitch that Bungie has removed.
The way it damages is by multiple attacks (obviously with extremely little time between them). It isn't a one-hit attack as it generally appears to be. It could also be said that the more time you're in the laser fire, the more damage it accumulates. If you're barely stroked by the laser (especially if you're in a speedy vehicle), you may survive. Evidence to this could be how you can shift the laser a bit during mid-shot (during the time the actual laser is shot).I have got this info from halopedia.com