Easy boosting method, alone, with only ONE Xbox
You can do this coop achievement alone if you have the following:
-Have an Xbox One / Series
-Have Xbox Gamepass Ultimate
-Have a phone or a computer
1) With your main profile, set your Xbox One / Series as your "Home" console so that any profile on it can access downloaded gamepass games without having to pay for another subscription.
2) Start Halo Wars on your Xbox One using a secondary profile.
3) Start Halo Wars using xCloud on your phone or PC with your main profile.
4) Invite your other profile, start mission one, speedrun it, unlock your achievement.
Using this, I was also able to get the 100 wins achievement in around an hour.
Works for any gamepass game if you need coop/boosting on other games!
PS: I played using an Xbox One and a computer. xCloud using Xbox Series, loading times were much shorter on xCloud than on my Xbox One !