Metal: Hellsinger

Just start up the Torment that requires you to kill 99 without dying. Start a train around the red crystals and shoot it when there's a group of the grunts around it.

Easy.

Don't tell me it won't work because it does. You're just not trying hard enough.

I didnt realise this was an achievement until I unlocked it.

So what I did was I had 2 Ultimate boosts, one for Paz (your friendly skull guy), and the sword.

I then was running around bunching enemies up, press Paz for his Ultimate (freezes enemies in place), then switch to your sword and then quickly press for your Sword Ultimate and swing in the direction where you've frozen enemies.

Simple as that! May take a couple of attempts but I was able to get this during Mission 2. You can probably get it anywhere as long as you've bunched up enough enemies.