To have any chance of getting through Halo 3 alive you’re going to need armour. Below is a list of the armour available and what the task is you have to do to unlock it.

Elite Armour

Unlock Ascetic Body - Get the ‘Up Close and Personal’ Achievement.

Unlock Ascetic Head - Get the ‘Steppin’ Razor’ Achievement.

Unlock Ascetic Shoulders - Get the ‘Overkill’ Achievement.

Unlock Commando Body Unlock ‘Triple Kill’ Achievement.

Unlock Commando Head - Get the ‘Overkill’ Achievement.

Unlock Commando Shoulders - Get the ‘Killing Frenzy’ Achievement.

Unlock Flight Body - Complete Tsavo Highway On Herioic or Legendary.

Unlock Flight Head - Complete Campaign mode on Heroic difficulty.

Unlock Flight Shoulders - Complete The Ark on Heroic difficulty or higher.

Spartan Armour

Unlock EOD Body - Complete Tsavo Highway on Legendary difficulty.

Unlock EOD Head - Complete Campaign mode on Legendary difficulty.

Unlock EOD Shoulders - Complete The Ark on Legendary difficulty.

Unlock EVA Body - Complete Tsavo Highway on Normal or higher difficulty.

Unlock EVA Head - Complete Campaign mode on Normal difficulty.

Unlock EVA Shoulders - Complete The Ark on Normal difficulty or higher.

Unlock Hayabusa Chest - Collect 5 hidden skulls.

Unlock Hayabusa Helmet - Collect 13 hidden skulls.

Unlock Hayabusa Shoulders - Collect 9 hidden skulls.

Unlock Mark V Head - Get the ‘UNSC Spartan’ Achievement.

Unlock ODST Head - Get the ‘Spartan Graduate’ Achievement.

Unlock Rogue Head - Get the ‘Spartan Officer’ Achievement.

Unlock Scout Body - Get the ‘Too Close to the Sun’ Achievement.

Unlock Scout Head - Get the ‘Used Car Salesman’ Achievement.

Unlock Scout Shoulders - Get the ‘Mongoose Mowdown’ Achievement.

Unlock Security Head - Get 1000 GamerScore points.

Unlock Security Shoulders - Get 750 Gamerscore points.

Unlock Katana:
When you’ve completed ALL the Achievements and found ALL the skulls Katana becomes unlocked.