Quantum Break

You'll get a few of these through normal progress, but if you're looking for an easy way to get them quickly, load up Act 5, Part 2. From the beginning, kill the 2 enemies and take the cargo elevator down. At the bottom, peak around the corner and shoot the red tank next to the 2 enemies inside. Once it blows up, both enemies will get killed and you'll get credit towards the achievement. Now, pause the game and "Restart checkpoint", to respawn at the bottom of the elevator with both enemies alive. Rinse and repeat until you unlock the achievement.

You can replay part of Act 1, Scene 3.

After you enter the library, and have to save your buddy William, a bunch of Monarch guys will storm in, wait a few secs until there's 2-3 coming down the right and start shooting the propane tank. You can make at least 2 enemies die in the explosion.

Now just press and reload the checkpoint.

Rinse and repeat until you hear a bloop. Takes 5-10mins.

Video https://onedrive.live.com/redir?resid=E3F3D85B1E14B4EC!85880...

How to unlock: Eliminate 15 enemies through secondary explosions. The secondary explosions may be of your making, triggered by outside influences, and/or initiated by the enemies themselves. Earliest opportunity to unlock: Act 1, Part 2.

There are many opportunities for enemies to be killed by secondary explosions either through your doing, the environment, or by their own hand. Accordingly, the achievement should be unlocked through normal game play. It can be unlocked earlier by causing lethal secondary explosions or maneuvering enemies so that they kill themselves with their own explosives. You can reload checkpoints just before areas that lend themselves to secondary explosions.

The secondary explosion may be caused by shooting a fire extinguisher, blowing up a car, enemies throwing grenades in too close proximity to themselves or their comrades, shooting explosive tanks, etc. The second and third enemies that you have to kill enter a door with a nearby fire extinguisher. If you shoot the fire extinguisher killing them, you will get your first two kills towards the achievement. You cannot replay this sequence because it triggers the next checkpoint.

The earliest convenient opportunity to unlock the achievement occurs when you open the door to Will's car a little later in Act 1, Part 2. You learn Time Stop and bullet stacking. If you target cars with Time Stop and stack bullets into that Time Stop, you will explode the car and kill enemies in the vicinity of the explosion. You may want to use the Achievement Tracker to track the Area Effect achievement. You can use it to take note of your progress in killing enemies with secondary explosions.

Reload the checkpoint before you kill the last enemy and, continue to explode cars through the use of stacking bullets into Time Stops targeting cars. Again, reload the checkpoint before killing off the last enemy and resume killing enemies by blowing up cars with the Time Stop/bullet stacking technique. Before long, you should unlock the achievement when you kill the15th enemy with a secondary explosion.

Note: At higher difficulty levels, the achievement is a little more complex. However, because you are using Time Stop, you are exposed only to moderate risk. You should be able to readily unlock the achievement on Hard Difficulty in the above manner.

A good point to get this is Act 2 Chapter 1.
A few minutes in the level, you encounter your first enemies in a storage building. When you enter the area, you face a group of three enemies with a destroyable crane construction above. Wait for the three guys to get together and shoot the crane, which will drop down with three explosive barrels. Shoot them to get up to three kills. You might get less, if the crane itself kills some guys. Reload from the last checkpoint and you are back directly before you shot the crane, rinse and repeat.