Coates flat out admitted fights and action were his weak point at the start of writing the book. For what it's worth, I do think he improved a bit in later seasons.
Though I didn't mind Tetu being taken out by the spirit spear. Magical weapons one hit KOing magical characters isn't all that uncommon. Would be nice if the spear was explained a bit more, but the showing itself was fine IMO. In RPG terms, human mages tend to be more controllers than Tanks. Dangerous because of all their tricks, but if you can just get to them they can often be taken out pretty easily.