if i move the ball to opponent battlefield can they use not so fast to prevent 2 damage to their units from the ball moving

No — Not So Fast can't stop that damage. It only counters an enemy spell or ability that chooses a friendly unit or gear, and The Ball's "Deal 2 to enemy units here" chooses nothing: it hits every enemy unit at that location by criteria, the same way "kill all units at a battlefield" doesn't target them (Rule 355.10.d).

Moving The Ball does put its "When you play or move this" trigger on the Chain, so they have a window to respond — they just need an actual damage-prevention or Shield effect. Not So Fast has no legal unit or gear being chosen to key off of, so the 2 damage goes through to each enemy unit there when the trigger resolves.