This should come quite easily but if you’re having trouble.
( This strat will work on any difficulty)
1864 scenario as the Yankees move all your units down one state so that on the next turn all the enemy units will advance and fight you. Merge your top left units into one army and buy a unit on the right side 1 state away from your army when it spawns in move the new unit to merge with the lower state.
Now the ai ( at least on hard) will attack your 3 units. The battles aren’t that strenuous if you can measure out a cannon shot early at about 75-80% power you’ll destroy a back line unit preferable the infantry if not doesn’t matter as long as your infantry are waiting for them to walk into your line of site you’ll win the trade every time at the cost of 1 maybe 2 soldiers.
Anyways after you’ve won the 3 battles they will have 2 forts and 1 possible army remaining.
Use your right army to attack the fort this will trigger a FPS game mode I can’t really give you much help with this part other then corner peak a lot as health seems to be hard to come by.
When the forts captured clean up the one unit that’s left if you haven’t already and you should get the win. You don’t need both forts although this is a good mission for the capture 5 forts but that would need to be on a new save.
With the 4 army’s destroyed and the fort captured you’ll win and it should be within a month of the start of the battle well inside the parameters for the achievement.