Survive 25 waves in each Exo-Survival map, beginning with Bio-Lab and ending with Riot. Completing each Map Tier will unlocks a new piece of zombie armor:

Tier 1: Zombie Gloves and Kneepads.
Tier 2: Zombie Exo-Skeleton and Footwear.
Tier 3: Zombie Pants and Chest Armor.
Tier 4: Zombie Head.

The entire set is unlocked after you have survive a total of 225 waves in Exo-Survival.