A 20% air blast is not the same as a 20% punch.
That's not really how damage works, because people don't actually function on hit points.
One of Nemu's power was specifically impact absorption. Someone who hits only 1.6% as hard as All Might probably wouldn't do ANY damage to Nemu, so they could hit it an infinite number of times and STILL not defeat it.
Or, ya know, ANY villain who's power doesn't revolve around physical combat.
Like, realistically, if Tomura Shigaraki wasn't a known villain, what would actually prevent him from just walking up to All Might on the street, putting a hand on him, and killing him instantly?