Well as he did said he wanted to end the orc in just one attack but completely missed.
I tried doing it actually despite not being a swordsman I did slash a branch with my whole strength and it made me feel exhausted. Now I know it might be incomparable but Raon continued to receive attacks despite not recovering so of course he might feel extremely tired despite it being his first fight and I think you're underestimating the strength of the orc enemy. Do you really think those attacks received by the mc are just puny little slashes? So it definitely make sense for Raon to be exhausted. Not to mention he's still in a teenager body, so of course he'll get exhausted.
I think they should do something about the techniques/powers. Him just standing while holding his sword up just to do 1 slash seems kind of boring.
And why was he so tired? It was his first fight of the day. So he shouldn't be that tired after using one skill to cut the Orc's arm. It feels like they just wanted an excuse for the others to help him out.