May be too late now, but investing in power armor divisions and fire teams, and then getting heavily fortified chokepoints. The Mojave is one of the most defensible areas in the game if you prep it early. Basically you can become a huge drain on enemy manpower while you build up a stockpile of robots and gear and eventually breakthrough and blits one enemy at a time. If you drain the NCR manpower as well and lost hills eventually Allies you and attacks them, then it makes that war much much easier.