Castlevania: Harmony of Despair

To unlock hard mode you must first complete chapter 6 on normal mode.

You will then have to complete chapters 1-6 on hard mode and the achievement will pop. It's not easy so farming for chests during your hard mode runs would be a good idea. You get a lot better gear during hard mode and new gear is available in the shops.

You don't have to do it solo, the achievement will pop on multiplayer co-op.

So, if you want it fast without bothering with being good at the game or grinding to get good items, you can hop online and join teams doing your last accessible chapter. You can also create the team yourself and do one chapter after another until you're done.

There are enough good players playing the game to get you through the 6 required chapters for the achievement to pop. You don't need to be particularly active during completion, just being on a successfull Hard Chaper 6 run is ok.