I'm not entirely against bullet sponge Adam Smasher, but I do agree on some additional gameplay mechanics to make him more fun to fight against.
But honestly, the one thing I'd want is to feel like I'm in a desparate situation when fighting him. Like this guy is top-tier shit and I should still be a relative rookie against him. I should be competent enough to stand against him, but I shouldn't feel like I'm equal to his abilities and experience. Like, maybe give the player the advantage of being able to outsmart him but don't make the player feel that they could beat him in a one on one fight, regardless of the cyberware, weapons, or quickhacks at their arsenal. Make them feel like they were able to get a one up on AS with some clever thinking or even pure luck. Plus, just running around the arena and shooting AS isn't much fun.
That's why I think some more environmental damage might be good. Environmental damage doesn't have to gate AS from dying or losing health, but it should be a larger part of the fight. Give me a chance to let the ceiling fall on him, have unstable "Cyberpunk-magic" energy batteries to explode, and whatever else Arasaka tower could have to hurt the guy. Don't make it something as basic as having AS charge into one like some Looney Tunes character, but make the player have to carefully lure him into place to damage him. Makes it feel more rewarding that way too. Hell, make it feel like our fight is leading straight into Mikoshi, like we are trying our best to get closer and closer to our goal but at the same time feel like AS is cornering us into the depths of Arasaka.
And even more, don't let me kill the guy. Let me barely get away alive and still feel like AS is a threat and on my tail while I try to get to Alt. Don't let AS just be this one dimensional boring big bad guy boss that I, a nobody, finish off: let me feel like he's an actual rival and I've become one of his. Legend against Legend. Cybernetic meat against pure machine metal. Make me feel like I've become somebody but still have a lot to go, whether it's to become something like AS himself or more like Morgan Blackhand/JS/Rogue, etc. Make me question whether if I have what it takes to become truly a legend, or even if I want to be someone like that (which could tie very well with the Nomad ending: leaving NC to find another solution or to live out my days as a nobody with family).
A good boss battle is ultimately 1) fun to fight, 2) atmospheric, and 3) thematic with story. If we want a better fight, we need to be focusing beyond the first point.