This achievement unlocks after you research and build your first Titan Armor. In order to progress through the research and get the option to research this armor, you must follow these steps:
First research the Carapace Armor. Note the requirements.
Requirements: Salvaged Alien Materials, 10 alien alloys
Unlocks: Carapace Armor (HEALTH +4), Alloy S.H.I.V.
Once you have Carapace Armor researched (it doesn't have to built), research Titan Armor and build at least one. You must have researched Elerium as well.
Requirements: Carapace Armor, 15 alloys, 5 Elerium
Unlocks: Titan Armor (HEALTH +10, poison immunity, flame immunity)
EDIT: As noted in the comments, there is no need to research the Skeleton Suit first in order to get this achievement!
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