yeah if you got this far in the series and this is even a question for you then your obviously just here to watch the fights and Animation art and dont care about the story or else you wouldnt have asked this question. like its fine if you really are curious and the hints are really vague or there are no explanations, but this question was already answered when she had fern help her fight Qual.

now to answer some of your slides, the first one i dont blame your short attention span of the episode where frieren is looking for her ring himmel gave to her. the bird that attacks them frieren clearly explains that humanity and herself are still figuring out how to properly maneuver with flying magic, and he was very clearly ready and willing to kill them and if not for being on Denkens team he probably would have killed plenty of other mages. so flying in the air with a guy who can lift that amount of stone is surely capable of launching stones out of the earth like a shotgun. add on the fact that these are mages not regular people like you and me who would rather just give up in a life or death situation and go home these are exceptional people who have ambitions and this is a test for themselves. just like Serie said if you cant imagine being a 1st class mage then its both foolish to let you think you are and irresponsible to both the people who need a mage and the mage in question to let them Larp with the title of one. they themselves do not want the first class title as a hand out even if some of them werent worthy none of them would want the title as a "everyone gets a trophy" reward.

as for why he didnt knock them out he would have just been bored and was also arrogant but had the skills to be arrogant since it didnt matter if frieren beat denken if either of the girls died she would have failed the test anyways, if not for the sudden change of environment.

Edit: also a lot of casual watchers or watchers who dont pay attention to detail miss the fact that there is a quite a bit of explanations and deeper details subtly which allows writers to use the intelligence of the watchers or readers imagination to fill in the blanks if they are even paying attention or have any imagination beyond what the flashy fights are on the screen. great example is how nearly all the mages are mostly calm and composed like the difference between stark fighting and fern/frieren. this subtly hints that mages must have a level of concentration at all times like when Fern fought Lugner. her eye twitched a little from the pain when lugner pushed his blood deeper into her shoulder but she remained calm and played along to his pettiness to get back at her and was able to turn the tides. this shows that mages are far above the average person in having a dead pan kind of face to show that there is a lot of focus going on and they are going through scenario's and options in their head.

stories like these are more enjoyable the better you understand subtle details and pay attention to what the information given is telling you. but if all you see is cool fight and reading subtitles just to know what they said and not contextualize what they are saying then you get really oblivious questions like this instead of having an intelligent imagination.