I’m finding the Guard easier than the Marines right now.
Leman Russes are tough, and good protection for a line of Basilisks, which facilitate holding the ground.
Guardsmen are expendable, although they do get healing, which is nice. You can pump those out quite nicely, and the damage adds up.
I just figured out how edicts work for the Imperial guard. Delicious. They’re done by city, so you can ramp up production of infantry in one, and vehicles in another,
Plus, having more than one city (I’m on 4 now) allows for a strong economy.
Specialise your cities and you can pop out many troops once everything is in full flow.