To first unlock INSANE mode, you'll need to beat all 8 intervals on a lesser difficulty. Once INSANE is unlocked you'll need to complete and entire interval without dying, restarting or going into Last Stand. I recommend Interval 3 for this particular achievement. Most of the enemies are melee focused and it's only at the immediate end of the interval do you fight any ATC forces. If you're still having a hard time earning this achievement play multiplayer and act as a reviver while you let your partner do all the killing and dying.