There are many special achievements in Sonic damage and clear the level.
What to do if you can’t touch this achievement in the Sonic Chat Generations reboot
Don't touch this
In Sonic Generations, avoid taking damage from the final boss and clear the level.
Personally, I recommend always keeping Modern Sonic, and using Classic Sonic to destroy rubble every now and then. Be careful not to hit anything, walls, and of course the attack itself, and you should be fine as everything is easy to dodge.