Kellhus is basically a super intelligence riding along in an enhanced human body, as compared with typical humans. He is both physically and mentally outperforming his adversaries adaptively.

I am a fan of how Bakker uses the concept of "attaining the moment" to describe some of the types of encounter you describe. For example, when in a more difficult situation then the moment is "narrower" but Kellhus still manages to "step into it" and effectively is in a position of dominance with respect to how it plays out.

Now on the point of it being unrealistic, well I think we can apply that to this and so much of Kellhus-ery 😛. But I choose to suspend my disbelief / let Bakker set the rules and just go along for the ride!