That cover made all the difference. This is Superman Special #10, which is a comic published only in Germany during the latter 1970's. You probably had the reprint from 1985.
The villain depicted on the cover - your mutated blond man - is Benny Baker, a sports writer who received Hulk-like powers from a scientist's formula. Benny took the name "The Fist" and planned on using his new abilities to become the new World's Heavyweight Champion.
The formula given to Benny is a type of "irradiated adrenaline". Therefore, the more his adrenaline pumps, the stronger he gets... eventually reaching a point where he is stronger than even Superman. When fighting in front of a crowd, the roaring & applause of the crowd further gets Benny's adrenaline going, making him even stronger. Superman eventually figures this out and uses it to his advantage.