Guys give him a break, it his first boss fight. He's not a isekai person who gets used to this kind of fight because of all the RPGs he played on his past life, he's a resident of this world who spent his entire life as a bottom feeder and expected to have a pathetic death, now after some months he's facing the strongest monster of this labyrinth, of course he'll have his doubts, he never fought a boss in his life.
If you give a rocket launcher to a boy, you still have a boy, but with a strong weapon. He has all the necessary tools to do the job, but his mind isn't there yet because he lacks real life experience of killing a very strong boss, this is why Ursula and Yuri and are constantly supporting him mentally, because his mind isn't there yet.
And this is realistic, put two equally skilled fighters against eachother, if those two are matched in skills and stats, then it becomes a mind game and the first one that loses control of himself, start doubting of himself, will lose the edge and probably the fight. This is the current MC, he has the specs, but not the mentality yet.