What make that Battle Droid Ultra? His rib gun and spurs?
If I recall correctly, he not only has the second set of arms on his torso, the spurs, a big ass shoulder cannon, and a flashy red dot on his forehead, but he's also about four times as big as a regular SBD. The story he was first shown in was in the Clone Wars Adventures digest comics (the ones done in the style of the CN cartoon) and Mace and Saesee Tiin made pretty short work of him. Apparently they have some kind of mass disruptor that makes it nearly impossible for a Jedi to use a Force Push on them.
The Annihilator Droid, btw, is based on discarded concept art from AOTC. Initially Lucas vetoed any other droids for the end battle besides the BDs, SBDs, and Droidekas, and the Annihilator was axed pretty early on. Then Lucas decided to have the art department make more droids (Dwarf Spider, Homing Spider, Tank Droid, etc), but for whatever reason, the Annihilator never made the film.