It was quite now. No one said a word, not even Kota who was behind Izuku scared. Izuku stood in front of Kota in a fighting stance ready to attack. As the two stood there Muscular stood across from them. All he did was smile at Izuku and Kota. Muscular was so sure that he'd win he didn't even get into a fighting stance. All he did was look at the two of them and smile.

Izuku: (whisper) Kota listen to me carefully. When I say to run back to camp ok.

Kota: but what about you?

Izuku: I'm going to stay here and hold this guy off while you escape.

Kota: but you can't fight him.

Izuku: while you're here I can't. Now listen carefully, there are more villains still in the woods so when you make your way back be very careful.

It was at this point that Muscular spoke up. Izuku and Kota stopped being to hush hush. Muscular ended up catching some of the stuff that they were saying.

Muscular: you should listen to him kid. It's about to get real nasty here and you probably don't want to see it. Of course it doesn't matter if you run or not, when I'm done with him I'll come for you.

As he said that Muscular's arm began to grow bigger as his muscle fibers began to pop out of his skin and grow. Seeing this Izuku's mind began to work. Analyzing everything he has seeing in order to find a way to win.

Izuku mind: those look like they're muscle fibers coming from his arm. Ether his quirk allows him to make new muscle fibers at will or grow his already existing ones. Most likely increases his strength. It might even increase his speed to an extent, not much though as it looks to bulk him up more. He is provided with more durability do to the extra layers of muscle.
Weakness. While his strength, speed, and durability are increased that doesn't make him invincible. His face, groin, and eyes are still a weak points. His exposed muscle fibers may be a weak point as well. Not to a direct head on attack but with more of a slicing attack to cut the fibers.
Conclusion. While being much stronger and definitely a tuff opponent he is not unbeatable.

Muscular: let's get this party started shall we!

Izuku: NOW KOTA RUN!!!

Kota: but.....

Muscular: here I come!

Muscular began to charge at both Izuku and Kota. It was at that moment though that Izuku vanished from Muscular's vision. When this happened Muscular stopped completely confused at what was going on. Unfortunately for him this made him drop his guard.
With Muscular's guard dropped Izuku came back into vision for the attack. While muscular saw this it was to late for him to do anything about it. Izuku landed a flying side kick straight into Muscular's nose. He wasn't done with his attack though. Izuku then immediately spun in the air and landed a spin side kick with the other leg. With one more turn Izuku landed a third kick. A front kick right in the nose again just like the last two kicks.
Izuku then pushed off of Muscular's face and landed a little ways back away from the villain. While Izuku did this Muscular stumbled back grabbing his face in pain. He didn't expect Izuku to be as strong as he was.

Kota: how did you.......

Izuku: Kota I told you to run now go!

Izuku looked back and shouted at Kota to leave. It was hear that Muscular regained his bearings and attacked Izuku while he was turned away. Before Izuku knew it Muscular was right on top of him with his fist drawn back. Muscular was definitely stronger then Izuku and now seems to be faster to. Still Izuku wasn't out of the fight yet.

Izuku mind: with his fist drawn back in that direction he is going to bring it down from overhead. I just need to roll.

Izuku did roll to. Not away though or even to the side. No instead Izuku rolled forward right to Muscular. At first Muscular thought that Izuku made a mistake. He found out the hard way that he didn't. Izuku came out of the roll and as he did he immediately shot his foot out and push kicked Muscular in the groin.
This hurt Muscular a lot. Even more then the triple kick to the face did. Muscular immediately bent over in pain. Unfortunately for him as he bent over Izuku kicked him in the face again. This time with a push kick though. The force from both the kick and from Muscular lurching forward made the attack hurt much more. So much that Muscular stumbled back and fell to the ground.
Izuku immediately got up from the ground. He looked at Muscular who was on his butt trying to decide weather to hold his groin or his face. Then he turned back to Kota who still hasn't left yet.

Izuku: see I can handle this Kota now go.

Kota: but.....

Izuku: listen to me, I can handle him but not if I have to worry about you as well now go!

Finally listening to Izuku Kota ran off to get back to camp. Izuku watched as he did until he heard Muscular getting back up. Izuku then turned to face Muscular who was now back on his feet.

Izuku: before this fight started your body language said that you were confident in winning this fight. Tell me, are you still that confident?

Muscular: kid I'm going to ripe you apart to badly that not even your mother will be able to tell who you are when they show her the body!

Hearing what he said Izuku just smirked. He then got back into a fighting stance.

Izuku: show me what you got tuff guy.

Muscular: I'm going to kill you.

Izuku: it wouldn't be my first brush with death.

Muscular: it'll be your last though

Izuku: we'll see.