Rise of Nightmares

Get 30 kills with explosive crates/canisters

This can be done in the same area as:

i.e. where the large rotating blades are in act 3 (if you haven't seen them already you will know when you come to them).

Instead of killing the enemies with the blades walk around the blades and through the door that has just opened on the other side of the room. Try to keep as many enemies alive as possible. You will see some red explosive crates, wait around these for undead to gather and when as many as possible are around you blow up the barrels by hitting them. This should get 6-8 kills each time if you wait for them to come in from the blade room as well. When you've used all crates or killed all enemies that can be lured just kill yourself on the blades and repeat, it should take 15-20 minutes from when you enter the blade room.

Note: On normal/hard the explosion from the crates can kill you but the kills still count, this sounds like it will speed the whole process up.

This can be done throughout the game but this may be useful for those who didn't notice many crates while playing through (like me).

When the enemies go to hit you they can blow up the barrels, these kills also still count so don't worry about that happening.

Feel free to comment any suggestions:
Thanks to BadBoyBungle for suggesting 'including amount of enemies needed to be killed'
Thanks to rican ninja89 for informing 'crate explosion causes death on normal+ difficulty and kills still count'

Nothing wrong with the guide by Subliminal Cuts, I just thought I'd add another solution that requires less walking.

Set difficulty to torture and load up Act 3 - Scene 2. Walk a few meters/feet forward and turn right. You see the red explosive containers and three creatures. Walk to the containers hugging the left wall and turn to the first creature. (Turn to face the first creature, because it seems to make he/she/it attack quicker.) He/She/It will hit you detonating the explosives, killing you and all three creatures. You respawn at the same place, so just rinse and repeat until you get the achievement. I got my cheevo this way, so I know it works.

(If you're wondering why torture and why let the monsters kill you: faster respawn. If you detonate the explosives yourself by kicking them, you won't get killed and have to walk upstairs to get respawned.)