Tell me, what armor does Mandalore have that stops Exar from taking his head clean off with a force blast? oh and how will he stop Kun from crippling his brain with his mind? Will he be able to stand up against a master of the lightsaber form that makes you an expert in balanced combat? not only that but Kun threw out the whole moderation stick with Niman and merely turned it into an extremely aggressive form when mixed with his immense mastery of Jar'kai and saberstaff that randomly changes length, one-on-one Mandalore goes down in seconds.
Yes, but just because one's a fine duelist doesn't mean they could stand up in, say, a bare-knuckles brawl, or a fleet-on-fleet battle. Mandalore was a brilliant tactician in addition to a beast in a brawl - and Exar always reliant on his lightsaber in a fight, and he was a fighter, not tactician.
And he may be a great tactician, but tactics can only do so much against an overwhelmingly aggressive horde of Dark Side infused maniacs with no care at all about anything but murder and blood.
The Mandalorians may be the most effective non-force using warriors in the galaxy, but Kun's Sith Empire was made up of a never ending force of Sith War Beasts(Which included abominations the likes of Terentateks) that Exar Kun will keep creating with that great knowledge of Sith Alchemy.
The most promising Force Users in the galaxy(which included the later known Darth Sion) and the Krath who were another warrior-like honourable people, but also completely fanatical and driven by the bloodlust shared from their force sensitive overlords, hordes like that can't be demoralised or waited-out, they also never stop.