Dear forums, I see this question more times than ANY OTHER… So I’m writing this to help YOU find out how to get those special armors.
For some of these armors you needed to preorder or other things you now can’t do anymore… 
1) THE HUNTER ARMOR SET (Spartan Locke’s armor)
Sadly in order to get this armor you had to preorder Halo 5 Guardians from GameStop:(
The Hunter Armor set incorporates Watershed Division’s latest warfighting technology primarily used by SPARTAN IVs. In order to unlock this armor set, you’ll need to have pre-ordered Halo 5: Guardians from GameStop.
2) Helioskrill Armor Set
So this ones pretty straight forward all you have to do is complete the legend achievement in HALO: THE MASTER CHEIF COLLECTION You get this by completing all the campaigns on legendary…
The first set of armor that many will be trying to unlock is the Helioskrill Armor. This special armor, which was designed by Sanghelli, is bred for warriors, and bloody combat. To unlock this armor set you’ll need to complete the “Legend” achievement in Halo: Master Chief Collection, which can be acquired by completing all the MCC campaigns on the Legendary difficulty.
3) NightFall Armor Set
So in order to get this all you have to do is watch Halo Nightfall on THE HALO CHANNEL. To get the helmet you need to watch all the Episodes but to get the chest plate you just need to watch the 5th episode.
Halo Nightfall might not have been the best Halo production ever, but the armor featured in the series is one of the most iconic of the Halo world. To unlock this armor you’ll simply need to head to the Halo channel and watch all five episodes of the Halo: Nightfall series.
4) Mark VI Armor
You need to earn the “Cairo Station” achievement in Halo: Master Chief Collection This can be acquired by completing the Cairo Station mission in Halo 2.
To earn this iconic piece of armor, made famous by Halo 3’s campaign, you’ll need to earn the “Cairo Station” achievement in Halo: Master Chief Collection. This achievement can be acquired by completing the Cairo Station mission in Halo 2.
5) FOTUS Armor
you need to purchase the Halo 5: Guardians Limited Edition 1TB Xbox One Bundle The bundle also includes the Vanguard Emblem.
The FOTUS is a variant of the MJOLNIR (GEN2) which is utilized by SPARTAN-IVs. FOTUS is an acronym for “Fist of the Unicorns”, which is why there’s a protruding horn coming out from the top of this armor set’s helmet. In order to receive the complete FOTUS Armor Set, you’ll need to purchase the Halo 5: Guardians Limited Edition 1TB Xbox One Bundle. The bundle also includes the Vanguard Emblem.
6) Teishin Armor
you’ll need to play the game prior to November 7th, 2015.
The Teishin Armor is yet another variant of the MJOLNIR Powered Assault Armor that is featured in Halo 5: Guardians. In order to unlock this armor set, you’ll need to play the game prior to November 7th, 2015.
7) Athlon Iccus Armor
you’ll need to have pre-ordered Halo 5: Guardians from Best Buy.
The ATHLON armor set features the most armor skins in Halo 5: Guardians. In order to obtain the ATHLON Iccus armor set, you’ll need to have pre-ordered Halo 5: Guardians from Best Buy.
8) Recon Armor
you’re going to need to purchase specially marked AXE holiday gift packs at Walmart.
The Recon armor was developed parallel to the Scout Armor with its main purpose being to offer stealth capabilities with little or no loss to endurance. If you want to fashion your Spartan in the Recon armor set, you’re going to need to purchase specially marked AXE holiday gift packs at Walmart.
9) Achilles Armor
Achieve level 3 in all of the Spartan Company Mastery Commendations’ Kill Commendations for the armor, and achieve level 5 in all of the Spartan Company Mastery Commendations’ Kill Commendations for the helmet.
- Helmet: The AI that lies within the branching lattice of the ACHILLES’ neural interface gnaws at the mind of its partner. No Spartan can stand alone against its urgings of unbridled excess, but through bonds of loyalty and duty they can be held at bay. - Body: Only Spartans who have mastered themselves, bound their wrath and passion within a shell of hyperdense steel fueled by starfire, are granted access to the ACHILLES. Lesser warriors will lose themselves in a storm of blood.
