I don't think you can get any higher aged troops when in IA or HMA. I think Arc could work with a diamond farm, but I would have it be the last GB to build after everything else is setup to the point of finishing GE each week, at that point Arc is the most effective GB for increasing city space through medal expansions. That being said I still wouldn't bother with an Arc for a diamond farm as it takes too much time to get it rolling and diamond farms, imho, should self level for time sake. Either way though the diamond farm that
@braeden12323 is building here is a one off and while it could be duplicated, wouldn't be efficient to play on multiple worlds.
I think the fastest way to get a diamond farm up and running would be to start in BA, RQ the helmets for a few weeks while self leveling Zeus and saving inventory fp's from the RQ's. Find a guild with a couple people that have level 80 Arcs get them to take spots on Zeus cutting down the cost. Once Zeus is at 10 move into IA, buy goods for Traz and put that down and level up to 3 to start, that should get you fighting through GE 3 with autobattle and rogues staying at about the same amount each week. Build a CoA at that point and maybe have 4-6 goods buildings and just enough housing to have those buildings, negotiate through GE4 each week. As you win FoY's put them down and when you get to a balance in goods from FoY's and the amount you're using to negotiate GE 4 each week drop the goods buildings and fill in with FoY's and WW's. From there it's just a matter of getting CoA up to 10 and Traz up to 10, that should give you the ability to autobattle through GE 4 each week and absorb the rogue losses. At that point you're up and running GE wise with a city that only has a Zeus, CoA and Traz. From there it's your decision on how to progress further or just stay at that point and enjoy the diamonds.