This is a cumulative achievement, all you gotta do is complete 100 parries throughout the game, you don't need to do 100 parries in the same stage/boss (Because that would be terrifying, even for Cuphead standards)
To do a Parry, just jump and press jump again in a pink object, you learn this in the tutorial of the game. You'll get this achievement with time, so there's no need to go for this one.