Pavise Guards (if those are the bosses based on cyberdemons) I'm finding take a really long time, yeah. Mainly, it's the enemies that shoot cannonballs since the scythe maps are absolutely filled to the brim with them, they're often at a distance, and in the best case scenario, it takes 5 point-blank shots with the thunderbuss.
As Blaz you could press the zoom button to supercharge your gauntlet and chew through them (though sometimes the flying reaper enemies wouldn't die. I'm not sure if they have some invincibility mechanic.)