At the beginning of the game, you will choose between Spartan and survival mode, then choose a difficulty. Pick Spartan mode and finish the game (either ending) to unlock this achievement.
In Spartan mode, you and your enemies both have more health. In general, this makes mutants more dangerous and human enemies less deadly. Combat will be slower, plan accordingly. There should be more supplies available, so try using military ammo (the currency) if things are too tough.