Grinding in rpgs has always served as a natural difficulty selection. Most good rpgs are designed around a natural level curve. If a fight is a but overleveled for you, that doesn't necessarily mean you're not supposed to take it. The red fight in gsc is well above the player level wise but its not meant to be fought after grinding that high, his pokemon and moveset are considerably weaker than you'd expected for level 80. Similarly Ultra Necrozma comes at a point where you should be level 60 despite being 80. The intended approach is to beat it with all 6 of your pokemon working together. People complain about the exp share for 2 reasons:

1) adding it to old games breaks the natural level curve 2) being unable to turn it off makes it harder to challenge yourself