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Nero was flabbergasted. He was absolutely stunned, and he wasn't the only one. Everyone almost immediately noticed that this fight was much easier than it should have been, but only Nero himself knew the reason.
The others could speculate. They could guess that the berserkers were still weak from Nero's flames - he'd suggested it might be a possibility earlier - or that the others who were fighting them had weakened it severely.
But the simple truth was that Nero was stronger. The increase in strength could both be described as significant and insignificant. It was a big deal because under normal circumstances, it would take him months of training to increase his strength this much. It should have hardly mattered because compared to a berserker, Nero was still a weakling.
For example, if prior to his power up, the sum of Nero's strength could be denoted as 100, then after the power up he was 120, or around that much.