Dauntless

The Timeweave Armour is a special armor set that is not tied to a particular Behemoth, and is unlocked through a Lore point on the Paradox Breaks map. In order to unlock it for crafting, you first need to reach Milestone XI on the Slayer's Path, and then purchase the Paradox Breaks as a destination. Once it's unlocked, you need to go there and then get to the large temple structure on the other side of the map. Just after entering that temple, before the little bridge over the dip, there is a purple pillar on the left. Behind that pillar is a set of discarded armor on the ground. Interacting with it will unlock the Timeweave Armour set for crafting back at Ramsgate. Just talk to Moyra and you'll be able to craft the armor. Each piece has the same crafting requirements, which are as follows:

250 Aetherdust(Obtained from salvaging Cells at the Middleman and breaking open Cores)
1 Cataclysm Shard(An Epic drop from slaying Rezakiri)
1 Galactic Shard(An Epic drop from slaying Heroic Valomyr)
6,000 Rams(In-game cash)
1 Stoneheart Shard(An Epic drop from slaying Heroic Rockfall Skarn)

Craft any of the pieces of the set and the achievement will pop immediately.