@SteaIthIess hard mode pretty much prolongs the battle (adding one more hit to the fight). I haven't done Death Egg Robot but I assume its the same for him. For shadows fight, he merely boosted a lot more and collected rings faster. Thanks for watching.
I don't get why people say that this fight is too easy or it's disappointing. This isn't the final boss anymore, it's the second out of 4(excluding rivals). They couldn't have made this the same difficulty as the original otherwise the next fight would be ridiculously hard.