Ronan's wrist was completely shattered after his fight in the semi finals. Karna tried to heal it using his light magic , however while he could numb the pain. Completely mending a shattered bone in a day was not possible with his abilities.
This meant that Ronan had to head into his next fight with a broken wrist and that to from his dominant hand , which meant that his odds of winning had went down significantly.
However despite the pain and the hardships that were ahead , Ronan was calm with a smile on his face as he assured karna that he had got this.
For Ronan the only insurmountable mountain towering above him was Percy and although he could be defeated by others , he did not fear facing them.
Alexander had raised Ronan and Percy fighting the soldiers of Avalon army since they were 5. Hence while everyone else in the world would find the disadvantage of build and insurmountable power gap to be a huge disadvantage , for Ronan it was a familiar feeling.