KMST 1.2.411 - Phantom

Given they seem to have less boosting skills to back up their main attack than the original holders of those attacks, how? Is it entirely the magic of combo attack?

If you can steal Continual Aiming, it's a permanent debuff on a monster, so it shouldn't be removed when you switch to Rapid Fire with Combo Attack. They have their own +30% damage and 30% Defense Ignore, and 220% Final Attack activated based on their own critical hit rate. They're definitely not bad DPS.