This is so disappointing when compared to the light novel that I felt like I have to leave a comment. I was honestly expecting this to be a fight that takes potentially 3 chapters as it has a lot of detail and cool scenes. First of all Suimei didn't lose in 2-3 strikes, he held out quite a bit until he was cornered and wasn't basically bullied like this. He had also unleashed his mana fully when introducing himself, alerting all the animals around making the princess feel threatened. During the fight he had to change the placement of the shield several times, and carpet bombing was the type of spell that was said to cover the sky, and princess dodging all of them made Suimei call her a monster and question why they summoned a weak hero in the first place if they had warriors like her. He used the sword when the princess got close enough and fast enough that he didn't have the time to deploy his shield, and the sword is made out of mercury.
Honestly this is a total butchering of the light novel. It has been getting worse and worse with every chapter release as they make important moments and fights shorter and simpler. Remember how long and true to the novel his encounter with white mage was in the garden? I think it is because they are close to axing the manga. The volume 9 was delayed by the publishers and volume 10 is still not out thanks to the publishers basically killing author's desire to write and even more issues.
I might take a look at this from time to time. But I have no expectations left. One of the most interesting parts about this story was the magic itself as it had its own rules, systems, and went quite in depth on how things worked. But in its current form it is just firing mana around and doing flashy moves. As someone said earlier, "but do stupid magic like making a sword and throwing energy balls".
Note: In novel why he doesn't use more advanced magic often is explained. Half of his magic can't even be used as his magic mainly borrows powers from legends and stars etc. While the ones he can use are greatly weakened. His family specialises in spirit magic where they borrow power from spirits like angels etc. They are basically the last lineage left on earth still using such magic as making contracts became harder and harder.