Dark Void

It's easier that it looks because the description is misleading.

You need to kill 10 enemies with a grenade that did not touch the ground yet - that's the explosion reffered to "fireworks in the air". Enemies aren't ought to be in the air at all. The grenade needs to be in the air while it explodes. Just hold the grenade in your hand long enough and throw it into a bunch of enemies.

Do it on casual. Enemies aren't that tough and they can easily die of a grenade explosion instantly.

Best done behind the cover or throwing grenades from a higher point of view.

I got this at the beginning of Act 3: Chapter 3, The Revolt. (Thanks TeLMo BR) At the very beginning you're in a room facing a big screen, where a fire fight will emerge. 3 flying fellas will be the first enemies you see, focus on them. Shoot them a bit first to soften them up for the grenade lob. I was on the easiest difficulty as well. Sometimes I'd only kill one, sometimes all 3, thank goodness ya just need 10 kills. Just finish the fire fight in the room and head down the hall where there is a check point, let it save there and then restart the level. That particular level only allows the use of The Liberator gun at the beginning, I was going to try the Magnetron gun trick but, it wouldn't allow me to use it.

Knights are the big flying bots. You can shoot them till you see blue smoke around them and then toss a grenade, this will kill them. The easiest way to get this achievement is to get the magnetron gun and zap normal enemies with it. They will float up temporarily. While they are floating prime the grenade and throw. 10 later it pops.

First off as mentioned by some true achievement posts the 360 achievement text is wrong, but it's correct for the PS3.

Trophy text for PS3 is: Kill 10 enemies by exploding a piece of dynamite in the air

If you do a firework kill correctly you get a special death animation where the enemy's head pops off and explodes like a firework with their headless body falling to the ground. You need to see this death animation 10 times for the achievement to pop. It only works on the basis type watchers units and their colour variates and no others.

You need to make sure that the grenade blows up mid air near their head to make this happen, if the grenade is too close however it will just kill the enemy out right, I got it down to being able to do it with 1/3 enemies, timing is a bit of a pain for this.

In short you need to hold the grenade and throw it just before it blows so it will blow in mid air. Once I figured this out the achievement was easy to get but it does take some practice and a few extra kills.

Remember if their head doesn't shoot off like a firework, that enemy will not count towards the achievement.

Would be great if someone could make a video of the needed death animation sadly I can't capture video to show you.

As people have noted in the comments to IxI Suicide IxI's solution, the achievement description is wrong. To get it, you need to kill 10 enemies using grenades that explode in mid-air.

The most straightforward way of getting this is to throw grenades when looking upwards in vertical combat. I got this playing in Hardcore mode and so I would weaken enemies by shooting them a couple of times with the rifle before cooking a grenade for a second and then letting fly (on Casual you can just throw the grenade without having to shoot first).