Master rounds are items that are dropped by bosses if you kill them without taking damage. You can collect one for each regular floor (doesn't include secret floors like a sewer). So you need to kill all 5 floor bosses without getting hurt at all in a single run to get the achievement. Focus on items that give you invulnerability and block bullets (like guon stones). This is not an easy achievement and you'll fail many times, but don't lose hope!