This is my post on the behalf of Mandalore. I have a sinking feeling this is for not, and that Grievious will in, but I have to play devils advocate here.
Mandalore just doesn't have the numbers to keep up with Grievous.
Neither did the SE, but they won.
Also, Mandalore was known to recruit other species into his armies. He did so when he became Mandalore and wanted to rebuild the clans. He also used politics to help build his army. Although this may not help to much(no outside parties) he may be able to get some key CIS member to join his side. He is also known to test his enemy to see if he would win a battle, and would not easily fall into a trap or other situation that he couldn't win.
Mandalore didn't lose. He almost never did. He tried to always win, even to the point of nuclear attacks just to make sure his of his victory. He sacked the entire world of Althir. He destroyed much of the planet Cathar (this is why they hate Mandalorians.) He nearly made Cathar extinct when he bombed their planet! What says he wouldn't do it again?
As I said earlier, he can destroy the super tanks with explosives and even if they cripple him, there were only 24 ever recorded.
Now to Cassus Fett- the man who turned the Mandalorians into a war machine. He standardized Neo-Crusader armor, and led the siege of Taris. He massacred the Cathar people, and tested the strength of Republic soldiers and ships with Mandalore. He was Mandalore's second in command, and an excellent fighter and strategist. Neo-Crusaders where at first a cult, but Fett began to spread the word to Mandalore, who encouraged it. He didn't simply conquer worlds, Fett made them Mandalorian worlds, and gave them his culture.
Fett was never even killed, merely presumed dead. He killed the captain of a Republic flagship and lived. He even became the most wanted man in the galaxy. Fett killed thousands of people simply for revenge, or honor. He used trickery in his tactics and was as ruthless as you can be. The CIS would know next to nothing about him either, as even the Republic who fought him thought he was a mere thug.
Alright, those are my arguments for Mandalore. Debate them if you must, but I will defend my post to the death!