I'm not an expert on NU, so take what I say with a grian of salt. Anyways, if you want to use a pokemon like Bronzor you want to try and find a specific niche it has that no other pokemon can do. To continue off of your Wartortle example, even though Bronzor has better overall defenfive stats, what keeps Wartortle around is the fact that it can use Rapid Spin, which can be very important given the amount of Fire and Flying-types present in that tier. Bronzor would have to have some kind of niche that nobody else has.
As just a defensive pokemon that can set up Stealth Rock, I really don't see a reason to use Bronzor when there's pokemon like Regirock, Torterra, and Torkoal that can all do the same thing but with either more attacking power, or more ways to support the team. Where bronzor might stand out is the fact that it can use both Sealth Rock and Hypnosis, effectively making your opponent play 5 on 6 for the rest of the game. However short of this, since Bronzor literally can't do any damage at all (base 24/24 attacking stats lol) I'm not sure there's anything it can do.
