I found air power important, the land you fight on favours the defender, so use the power armour to air drop in, CAS to support them while you auxillaries press the main line. I got ♥♥♥♥♥♥ up a few times and was unable to save my power armour from being surrounded, but after a few try's I managed to win, just be patient and don't try to end the war fast, take it easy, bleed them out and take the ground you need.
obviously early game take land really fast, espeically to the north, try to take the whole border of Nevada before the war with the NCR and you should have the man power. Purists are hard due to their lack of legitamacy.

I've found if Rangers go NCR do not take Yakuza, so you have a buffer between yourself and the Rangers. Then you can push down to Shadysands and hold up there with a small defensible front while you push up and eliminate the Rangers. Then you continue the conquest of the NCR.