Successfully complete the game under the Hard difficulty setting.
In addition to the three difficulty settings available from the start, there are two unlockable difficulty settings.
Destroy the hives as soon as possible, as enemies will continuously spawn from them.
Let the enemies come to you. After destroying a wave, go around and take out the enemies blocking the radar. You can also use the tank like a turret if needed.
Each weapon (including grenades and the whip) has 2 upgrades that count as additional weapons. The upgrades carry over even if you start a new game on another difficulty (on the same save spot).
Complete the indicated task to obtain the corresponding weapon upgrade. Each weapon has two upgrades that are treated as additional weapons. They can also be used with a new game on the same saved game file.