This achievement is not retro-active!
You must complete 10 more Syndicate missions in addition to any you have completed previously. It also seems to be potentially a little glitchy, but the in-game counter should reflect your correct progress.
(to view the in-game counter, when in your ship, press
, select PROFILE, select MY PROFILE, then view ACHIEVEMENTS--find "Gunning for Promotion")
When and how can I do Syndicate missions?
Syndicate missions are available once you reach Mastery rank 3 and are found in the navigation system on one of the tabs at the top right. You must also have the mission node unlocked in order to do the mission.
What are Syndicate missions?
Syndicate missions are special missions granted by the various Syndicates in Warframe, that upon completing successfully you are granted Syndicate standing.
Syndicate standing has two main uses: leveling up the Syndicate to reach higher rewards and purchasing Syndicate rewards.
Which Syndicate missions count? (as of November 2016)
Currently any Syndicate missions given from the 6 primary Syndicates; Arbiters of Hexus, Cephalon Suda, Red Veil, New Loka, The Perrin Sequence, and Steel Meridian.
Cephalon Simaris scanning missions do NOT count.
Do you collect the hidden Syndicate emblems in the missions?
Syndicate missions also have hidden emblems around the missions map, almost always 8 of them. Gathering these are really helpful for getting extra standing and they come in 3 varieties--common, 500 standing; uncommon, 1000 standing; rare, 5000 standing. If you have loot radar it is extremely helpful for finding these little emblems.