20% Izuku (
Low 7-B) was fighting evenly against Flect and wearing him down after less than 1 minute of fighting.
Flect can be defeated as long as you have someway around getting your attacks reflected back at you. Enel won't be hurt by his attacks being reflected, as such he can handle it far better than Izuku did, who was just taking the damage and ignoring the pain.
Enel's attacks would be more effective than a normal punch as well, since they'd be electric. I'm confident in saying that if Flect fought Enel for 5 minutes his Quirk will be worn down completely and will lose. Heck, I'm confident in saying 20% Izuku could beat Flect as well, he just didn't because he was on a time limit due to the bombs.
Which is why Izuku chose to use 100% when there was only a few seconds left on the clock.
Weakness: "His Quirk has a limit to the amount it can reflect, and can be surpassed if enough strength is used. It can also be worn down over time with repeated blows of significant power, though how long it takes for his Quirk to run out in this case is unknown."
While the exact time is unknown,
Izuku's attacks were already reaching him and hurting him. You can see his Quirk tried and failed to completely reverse that impact. Heck, in order to land that punch Izuku smacked away Flect's arm. Once again, you don't need to have High 6-C level of power to reach him.
Low 7-B can wear him down after like a minute of attacks.
Flect has zero way of harming or wearing down Enel, making this fight a one-sided stomp.