Gears of War

This achievement requires you to kill 100 people with the Torque Bow in ranked matches. The Torque Bow is a slightly odd weapon that requires you to charge it before your fire. If you hit the target, it's a one-hit kill. However, if you prematurely fire it, you can very well end up killing yourself. Also, you can stick the arrows to walls and damage foes with the explosion. The active reload for this weapon makes the charging time reduce by about 30%, so you can kill enemies faster.
The drawbacks to this weapon are 1) Limited ammo and 2) Easy to spot. You only get 6 shots to play around with, unlike the Longshot's unnecessary 11 shots. Also, the Torque Bow makes a lovely charging noise and emits a very obviously glow. This means that enemies are very likely to see or hear you if you're trying to kill them.

A small strategy to use if you're having trouble getting Torque kills is to play on Execution with 5 second bleed outs. Melee someone down with the Torque Bow and fire it at them when they are downed. When they get up, the arrow should explode and kill them.

Maps with the Torque are: Canals, War Machine, Mausoleum, Tyro Station, Rooftops, Bullet Marsh, Subway, and Process.

Some maps are good for Torque kills while others are a little more difficult.
Canals:
The Torque is smack dab in the middle of the map. and is usually a hot-spot for carnage. If you can grab it and run away, then you can probably score some kills. Otherwises, it's a death trap. Beware of snipers.
War Machine:
This map was two major "go-to" spots. One being sniper and the other being torque. Torque is kind of in a bad spot since you'll probably be rushed by people with shotguns and someone is bound to throw grenades in your direction. Also, the sniper is faster to get to so if someone on the other team has it, they might kill you before you kill anyone with the torque. There is also a fair amount of cover and it can be easy to lose someone, so the torque is a tricky weapon to use here.
Mausoleum:
I am pretty sure this is where I got my achievement from. The map is circular, with sniper being on one side, torque on the opposite, and boom shot in the middle. Getting torque can be pretty easy depending on how many people jump for it (Sniper and Boomshot tend to be more popular weapons to acquire). When you get the torque, you can try flanking around back to sniper or head towards boomshot. I tend to try killing people by boomshot, but you just have to watch out for sniper (who is pretty much hiding right where the sniper spawns: on high ground covered with dense fencing.
Tyro Station:
This map lacks weapons, and it might be hard to get Torque since everyone wants it. If you don't spawn closest to it, you probably won't be getting it. If you do get it, you'll have to worry about the enemy with torque across the train tracks. That person will possibly be your only real threat unless you're the last man alive and you have two or three guys rushing you at Torque spawn. Overall, though, it's actually a decent map to get a few kills on.
Rooftops:
You won't be getting too many Torque kills here. The weapon is placed smack dab in the middle, where you can easily be killed from any variety of other weapons. There are also bunkers to the sides of torque which could house at least one enemy shooting you to death.
I recommend going for the torque when there are only a few people left on the other team. This definitely isn't a weapon to grab shortly after the match starts.
Bullet Marsh:
Out of the few matches I played here, no one had Torque. It's in a really bad spot: Under light. Someone can destroy the power generator and prevent you from getting the weapon. No one really plays on this map anyway, so it's nothing to worry about.
Subway:
Torque is downstairs, in a subway car, in a pretty safe location. Getting it depends on where you spawn. If you spawn outside, you probably won't get it. Spawn inside and you have a good chance at getting it. You should get a few kills with it here, just watch out for sniper.
Process:
This is actually a good map. Teams will split up for torque, boom, and sniper. Torque tends to be a popular spot every now and then, so you will run into 1 or 2 people depending on how everyone else is playing. You'll have to watch out for Boomshot right above you, but if you get the torque before they get boom, you can net yourself an easy kill. Again, just watch out for sniper and boomshot if you didn't kill them already.

If you are suicide boosting for the Seriously achievement you can save yourself a weapon boosting session with this method. Please look at the details of suicide boosting in the achievement guides for Seriously. This is not a forum on what method is best for boosting Seriously. If you prefer suicide boosting this method will help you.

Make sure you are suicide boosting on the War Machine map.

Before the session starts, ask to use the torque bow. Then use it.

If it doesn't unlock after exactly 100 kills, so what? you've got 10,000+ chances to get it right.

This method will save you a weapons boosting session and anyone who has boosted Seriously knows that every hour counts!