Normally the army's best shot would be to overwhelm him with firepower, but as the Gungans don't have any rapid projectile weaponry, they can't do this. They can only try to surround and dogpile him, and then it's just Sidious vs. the B-team for a longer period of time. At that point numbers hit diminishing returns; they can only attack him from so many places at once.
This is assuming Sidious engages them in melee and doesn't just break their minds, TK them, fry them all, etc.
It probably isn't very much in the "spirits" of the movies for a Force user to solo an army, but with Legends and even Disney canon Sidious would crush them pretty easily.