Sword Master is one of the few NPC summons worth their salt if for no other reason than he plays very defensively. Having him for the Champion Gundyr fight at a low SL run makes all the difference since he serves as a perfect distraction. Wait for Gundyr to aggro to him, land a spam chain of R1 before he's pounded into the ground, then let Gundyr chase you so Sword Master can heal. 5-6 rounds of that even with a +0 weapon and it's over.
Night and day difference to trash like Sirris, who is aggressive as **** and then tries to heal herself with a 5 second miracle 2 feet from a boss charging up their hydrogen bomb attack.