How can the Jedi "combine all their power"?
-- They aren't all going to be in the same room; the Temple is vast. As Illustrious puts it, not every room is as big as the Chamber of Moria, which would be the necessary size to house ten thousand Knights. Some are going to be training and sparring, others debating and dicussing, still others in deep meditation or in rest, etc. It's not as if thousands of Jedi are going to stand there in the Hall or at the Gates waiting for Ragnos and his battalion of droids to blast down their door.
-- He was that great. He was stated to be 'the most powerful of the most powerful', the ruler of the Ancient Sith Empire at its peak. He ruled over titans in the league of Naga Sadow, of Ludo Kressh. He vanquished the former Dark Lord, Simus, who was the most powerful Sith Magician and warrior of that time. Sith Lords who'd make Yoda shiver governed planets under his rule. And, oh. The two most powerful beings alive after his death feared his damn spirit. Don't talk to me about 'that great'.
-- As I said; he wouldn't be fighting every single Jedi in the Temple at once. And even if he did, the greatest amount of Jedi fighters that would be able to simultaneously attack him is at most, five. He could likely cleave them all down in a stroke or two, brandshing a mighty sword like that, one that is forged and blessed by Sith Magic, the Dark Side itself. Not to mention that, even if a large crowd of Jedi were to attack him, his HK droids and the IG-88s would throw mines and grenades, shoot rockets and blast wrist and back launchers, killing off dozens of Jedi at a time. And that's not even taking into account the Mandalorian Rippers, flamethrowers, heavy assault rifles, heavy blasters, Pulse Cannons, etc., etc. There are maybe twenty or thirty Jedi who, on their own, could defeat even one of these droids. Now they're up against seventy. *cough*owned*cough*
-- Wait a second. . . Marka Ragnos, Dark Lord of the Sith Empire in its peak, Bane of Simus, with a damn spirit that could order around someone who imploded stars, would run out of Force energy against a bunch of saber-swinging nimrods who'd get shot down by a lone bounty hunter? Tsk. Almost every single Jedi at that time uses Niman, the 'Diplomat's Form'. They aren't suited to fighting. A Jedi Master, a Council-level warrior, was taken out in four shots by Jango Fett. Those who aren't Niman practioners are younglings tarining in Shii-Cho, lesser Padawans and Knights, beings that aren't fit to wipe the filth off of Ragnos' boots. There are but a scarce few who can practice other forms with any kind of recognizable proficiency.
- Yoda -- Ataru
- Mace Windu -- Vaapad/Juyo
- Anakin Skywalker -- Djem So/Shien
- Obi-Wan Kenobi -- Soresu
- Kit Fisto -- Shii-Cho
- Luminara Unduli -- Soresu
- Cin Drallig -- Not sure, but he's definetely damn good at something.
- Depa Billaba -- Vaapad/Juyo (To a greatly lesser extent than Windu.)
And out of the above Jedi, none outside of Yoda, and maybe Mace Windu, would last twenty seconds against the leviathan of power that Lord Ragnos is.
So, to sum up this post; Ragnos and his elite, Jedi-killing droid troops, would waste the forces of the Jedi Order.
Although, to be fair, there is one being who could vanquish Ragnos, and with some ease as well. The Lord Artoo, for whom no words are great enough to describe. . .