Gears of War

This guide is pretty much meant for people trying to get this achievement without boosting.

The Hammer of Dawn is a somewhat difficult weapon to use since aiming it can be a little tricky and glitchy. Pretty much, you have to charge the beam for about three seconds before it fires. If the tracer beam is interrupted the countdown will reset. I would recommend not aiming the tracer beam directly at any target or, if they're moving around, near their feet. Try aiming the beam near structures or at something above them to prevent the Hammer from being interrupted.
Another thing to note: The Hammer's beam gradually gets weaker the longer the charge is held. After the initial blast (when the beam is a one-hit kill), let go of it and start anew. Since the Hammer has infinite charges, you will not have to worry about running out of ammo.

Another method you could try doing is downing an opponent (if you're bashing with the weapon, it takes three melee strikes to down instead of two), stepping back, and attacking them with the Hammer's beam. This is less effective on Execution than it is in Warzone. This achievement cannot be earned from kills in Annex.
Maps with the Hammer of Dawn:

Rooftops:
A personal favorite of mine, Rooftops has two Hammer of Dawns (one just outside each spawn in a little hut). I found it very easy to gain kills on this map when I was able to play on it. The trick is to try aiming at corners and pipes above the enemies. It will be hard getting kills with this weapon in the middle of the map (Torque Bow). Another benefit about using the Hammer on this map is that it will be difficult for other players to try killing you right away (unless your opponents include someone who knows how to use the Hammer of this map).

Gardens (DLC Map):
This weapon is on either the middle left or middle right of the map (depending on what side you spawn on) near a tree. While I have not seen many people use the weapon this particular location is a popular killing zone. I often found it difficult to even use the Hammer of Dawn on this map. If you do get it, you'll have to beware of flanking players and for people using the Longshot.

Tyro Station:
The Hammer of Dawn is located in the middle of the map, between the Torque Bows and in the way of the train. If you're capable of getting the Hammer, it can be a very useful weapon on the map since there is little vertical cover (unless you're by grenades) and the map is small. If the enemy has Torque Bow, though, you will have a difficult time to using the weapon.

Fuel Depot:
Out of all the maps this one is probably the most popular. The Hammer of Dawn is located in the building in the middle of the map. This weapon is handy since the map is large and open, and can even be used to blow up the gas tanks near the outdoor spawn (only once per match). The downside is that it spawns in a spot where it is very easy to get killed. Usually there is an enemy headed to this location as well and there is also an explosive gas canister by it which can be used against you.
If you do manage to get the Hammer, be mindful of snipers. You can use the weapon to flush out anyone in hiding unless they are inside the middle garage or inside the hangar spawn.

The other solutions are adequate, but this solution offers an alternative strategy to BOOSTING the achievement. Best done with a group of 6 people, on Gardens(a map from the free DLC) in Warzone with 19 round matches. I can confirm that doing this method, you will unlock the achievement after 100 kills. GETTING KILLS IN GROUPS WILL NOT HAVE A NEGATIVE EFFECT.

Cog #1 is going to head to hammer (located in the center of the map, behind the big tree, which is directly across from where the boomshot spawns) while all locust head to a designated kill spot (best is right outside of the roof, between the hammer spawn and the "kill team" spawn). In the meantime cogs #2 & 3 are able to be afk for the match (which should last between 10-15 minutes). cog #1 will get 3 kills/round X 19 rounds = 57 kills/match. Then rotate to locust #1 while locust #2 & 3 are afk for the match. And so on and so forth.

Doing this for the entire group, then switching to 15 round matches for the second set of kills will net each person the achievement, baring any dropped connections.

Total time to do this for 6 people will be about 2.5 hours, assuming no one unlocks it on their first set.
It is best to make sure the AFK people remain within mic range, just in case there are any dropped connections or other problems.

Please comment if you disagree or feel that this should be modified in any way.

Boosting is the best method to get this achievement as it is glitched and requires more than 100 kills to unlock it.Fuel Depot is by far the best map to acquire this achievement on. The hammer is inside the main building at the back and if your not boosting it can be hard to get without the opposite team trying to get it too.
When you have it you can only fire it outside but dont not to be outside to fire it. You should fire at the ground rather than the players themselves and as it takes a couple of seconds to fire, you should know where your enemies are at and try to hide before firing it. You can kill more than one person at any time but as it is a long range weapon then you shouldn't stand too close to your enemies when firing otherwise you might get yourself caught in the fire.

we just finished completing this achievement and we figured out a great and faster way to do this. Go on Rooftop, Warzone, 19 rounds. Each team is going to take turns. COG will get two kills one round and locust would get 3 and you would switch back in forth. After one game you should have 93 or 92 kills. One game takes about 32:00 mins. This is with 6 people. After you have completed 3 games everyone should almost have the achievement. Start up the last game with 9 rounds and everyone takes 3 turns so everyone will have over 100 kills and it will pop. It shouldn't take you more then 2 hours. Good luck!