Castlevania: Harmony of Despair

A dual crush is done by standing next to one other player and pressing the RT button when it appears on the screen.
You need to make sure you have at least between 60 and 70 MP for each character, Sometimes I needed 60 others a bit more.

I simply got it against the first boss. Got in a CO-op two player session and just beat him up until he jumps and breaks the floor, falling to the bottom.
Then kill all his cronnies down the bottom, then you and your buddie perform either 2/3 dual crushes and once it dies. Ping goes the acheivement.

Note: - You get different dual crushes with different combination of characters. Some appear to be stronger than others. Personally I prefer the dual Alucard one.

It takes 70MP(the green bar.) for each player to perform a dual crash.
So two of you can equip two sage ring/chaos ring for a faster recovery of MP.
Simple fight the first boss until it jumps and breaks the floor.
Then just repeat perform dual crush.(Maybe you can hit it 1 or 2.)
It takes 2~3 times to defeat it.

I would like to add that you have to accept the Dual Crush and defeat the boss to get the achievement. What I mean is, my first attempt at Level 1 boss, I requested the Dual Crush and when it was killed my friend got the achievement but not me.

Make sure you are the one who presses right trigger last out of the two, and have the attack kill the boss.

*Maybe my game just glitched and some others got the achievement for both players. Let me know if that's the case and I'll edit it in.*

Thanks

A "Dual Crush" attack is done by standing next to another player and hitting "RT". An area effect attack will result and cause moderate damage. Both players must have a large pool of MP available to execute the attack.

This is probably easiest on the first level boss. Battle him until he destroys the first floor. Continue to do battle on the second level for a short while, but stop after you've dealt a few hundred damage points. At this point execute the Dual Crush. It make take a few times, but be patient and you'll be able to bring him down.