The Imperial Guards being after the Tusken Raiders is a bit odd, but I figured that they're well-trained, and may have specialized equipment.
I'd gauge, from the fear that the Jedi seem to have of droidekas in TPM, that they're pretty well-protected. As we haven't seen a lightsaber blade come up against their shielding, we can't guess at the exact damage it would do, but we can guess it would be minimal, as no Jedi has yet tried to attack it with one.
I'd bet on the Droideka breezing through the first three, being put to the test in the fourth, and finally falling at Jango. He has a variety of weapons for a variety of situations, and his high mobility increases the difficulty the Droideka will have in aiming. If the Droideka catches him with his guard down and manages to kill him, it may or may not be defeated by the Tuskens -- it depends on if their rifles can get it or not. If it gets past that, the Guards will definitely overwhelm it, with their unsurpassed skills and good weaponry.