The difference in power between rank 3 (got no diff by mc) and rank 2 and 1(stated to be dangerous to fight by his parents) is massive. If he easily beats this beelzebub which I assume is rank 1, then there won't be any more threats of the mc losing or dying, there's just no more suspense
Stuff like this is why a story with an op mc is hard to do (mostly isekai), an isekai that have an op mc that's done right that I could think of atm is Overlord and Tensura, with Overlord Ainz is a mage with a lot of weakness, he's not the strongest in that world, heck even some of his subordinates are stronger than him, with tensura Rimuru is weak at the start but slowly getting stronger, and the main suspense isn't on Rimuru being able to beat the bad guy, it's about his city, and when we saw that Rimuru is strong (like beating the orc lord) it was revealed that there is a group of demon lord who is much much stronger than Rimuru (we don't know if the demon lords are good people or not so we are in suspense whether a demon lord could an be enemy to Rimuru or not)