Bad guy : The MC is completely outclassing us.
Also bad guy : I copy the MC because he's the best form to copy.
Bad guy's conclusion : We'll win against MC.
To be fair, Yuuki isn't that good of a fighter. A more experienced and skilled warrior could beat him. That is, if Yuuki still doesn't take things seriously. He can even manufacture those sacrificial talismans, so what prevents him from carrying a whole wad of them with him? He would be practically immortal. Of course Yuuki isn't necessarily alone, either. Ul and Near ought to be with him most of the time, if he knows what's good for him.
The bad guy, Krom, on the contrary, seemed very used to fighting and using his power to the maximum effect in a fight, so the sooner he fights Yuuki, the better for him. But he's probably too arrogant to realise that. Not to mention he just wants to cause chaos, so killing Yuuki holds no inherent value for him.