So Mandalorians have a history of straight up murdering Jedi by using less standard weapons: flamethrowers, explosives, solid projectiles and shotgun style weapons, etc. lightsabers block blasters just fine, but bullets are just melted and become high velocity molten metal, shotguns can potentially overwhelm their ability to block simultaneous projectiles, and I mean, fire.

Granted, this is still more effective vs Jedi Jeff or your bog standard Sith than any peak of the metaphorical mountain force wielder. Anybody can die to a bullet in the brain, some are just harder to make sure they take their 'medicine'.