It's less he "didn't" and more he "couldn't". The thing about Karna is that he's very inefficient in terms of energy consumption. His fight with Sieg shouldn't be taken as his regular performance at all since he was directly hooked to the Grail then. I'd say a better fight to use as the model for "average" Karna is his fight with Vlad in Romania (where he used Prana Burst very sparingly exactly because he was worried about draining his Master dry).
Also, I'd say Cú Chulainn can take him in melee.
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It doesn't seem like much of a feint, though, it seems to be a raw power-style attack.