Speaking from experience, a good excavator is worth their fees. That rent a laser level from HD or Lowes with a transit stick, place your pavers under the uprights and check the level as you go adjusting it. But again like I say, do a search on YouTube or go to a reputable above ground pool store online and watch their installation videos over and over, you will see how the pros do it.
You can use sand for the bottom, or like Jamison said you can use foam for the bottom, and you can buy the foam cove for the inside where the liner butts up "this is a critical part of the structure of the pool" just like it being level. Or you will have issues.
P.S. It's hard work but well worth it, just dont get in a hurry and take shorcuts, you could damage the pool or injure yourself.