Step 29 during the Avenger's Reassemble campaign requires you to make "Final Preparations" before advancing. This consists of killing specific enemy types and picking up mission specific item drops. Most are easy; being completed following one or two missions. Prime Synthoids proved the most difficult to find, fortunately you only need a single item drop from them.
I finally found Prime Synths in the first mission of the Hulk's Iconic Mission Chain, Code: Green. During this mission Prime Synths spawn while you are destroying the servers in the AIM facility and the boss, Theta Team, has two Prime Synths in it. I'm not sure what the drop rate of the item is but I found it after the first Prime I killed.
If you've already completed Code: Green, Daniel Hollis from GameSkinny claims Prime Synths can be found on the missions "Our Town" and "Communication Nexus" both in the Utah Badlands. I played both and did not encounter any Prime Synths in either mission.
Once you collect all the items it opens up a mission on the Eastern Sea Board. Recommend taking a flying character to make the boss battle significantly easier. Nothing in this mission is difficult. Once complete, the achievement will unlock during the following cut scene.