The Captain does have a mission in Triton Flats. He is up near the Lunar Junction. Thanks to Dylan XBA for reminding me about this missing part. Also thank to Kanchanaburi for suggesting this is a two part or sequence of mission for The Captain.
You get this mission in Vorago Solitude southern most area. You a have to defend the Captain as he stands at attention. You have to kill 15 bad guys grab his arm support and hoist the flag a few times. Enemies are at level 23-24 so be careful, a mixture of sergeants and soliders from lost legion. Once all that is done pop goes the achievement.
**Edit - may be glitched**
I do not know how to solve this so if anyone does please let me know and I will update this