Shagard is actually talented and trained a bit before his nightly boot camp with Serena and Tigurd. As Serena noted in this chapter, page 3 (in case you missed it).
To summarize, Shagard was never exposed to fighting dirty, himself lacking confidence, and not maxing his physical potential (eating well and pumping iron consistently for example). We'll find out more in the upcoming chapters and what it all means though

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The manga skipped a few details,
but the training went on for weeks in the LN, giving him enough self-reflection and experience to confidently and competently fight in the next chapter - with more to come as a consequence of it.
But look forward to what will be coming, I don't want to spoil the plot and results just because I read the official english LNs.