As the King of dragons Acnologia is known as a pretty fierce opponent far and wide. It’s just very difficult for anyone to hope to defeat such a foe. In his dragon form he was known as a foe that could not be beaten. Sieghart does not have strong enough magic to seriously wound Acnologia in the slightest. The difference in their abilities would be readily apparent as soon as the match began. When the abilities are this one sided there isn’t much you can do in a fight. Acnologia wins.