VANQUISH

You will be introduced to the KNRB-0 Argus robot for the first time on Act 1-3 (Giant). This huge robot comes into 2 phases. The first phase, it will be four-legged. Shoot at the yellow glow on the arms and legs, destroying bits of the core in between when it's exposing it. After destroying the core, you will get a QTE.

In it's second phase, the KNRB-0 Argus robot will be standing on 2 legs. Again, shoot the yellow glow on his arms. Now, looking back at the achievement's description, are 2 words of great importance: "head" and "back". You won't see that same yellow glow on the head nor the back, but if you aim at it, you will see it also has a health bar. Now destroy both head and back. The order is of no importance. You might shoot the back first, then the head, or vice versa. It really doesn't matter. Once all these are destroyed, destroy the legs to expose the core a couple more times. As soon as you destroy the robot, the achievement will unlock.

If you didn't get this throughout the game, then you could try this. Select Act 2-3 (Nightmare) on casual difficulty after having beat the game. Enter the building, destroy the few enemies, and once you get out of the building, you'll be in the open again with 2 of these KNRB-0 Argus robots. One will be four-legged, the other one in the back will be two-legged. Go for the one in the back as it's already transformed into its second phase. Try to get weapon upgrades along the way. If you happen to die, reload last checkpoint. The reason why I didn't pick the one from Act 1-3, is that if you die when the Argus is in its second phase, you'll have to go all the way again to get it transformed from 4 to 2 legs.