Choose Mechanic Baird for your 50 waves for Unstoppable force and Horde Natural and build/repair fortifications whenever you can. This should come easily while going for the other 2. Easy mutators do not affect how much money you earn, nor do they affect how much of the money you spend 'counts' towards the achieve. The only thing that increases $$ earned is difficulty and it isn't worth the trade off. We did it on Casual with 3 easy mutators and all 5 of us had this by wave 50, with plenty to spare.