Pikachew
Please stay away from Probopass. Its an awful steel type thats hardly used for a reason. It has a quadruple weakness to both ground and fighting moves and its slow. Youd be investing almost half of your max EVs into its horrible HP stat, hoping that the defenses will be good enough after splitting your remaining 258 EVs.
Then youll go into battle where everyone and their mother will destroy it since its not a good attacker and a terrible wall with a limited move pool and no means of recovery.
Pair it with Tyranitar or Hippowdon. The rock type special defense boost from sandstorm goes a long way. I know OP was looking to make a steel monotype, so that not relevant to them, but just for examples sake: 252 hp / 252 spdef +nature. You end up with 324hp and 657spdef 438 x1.5).
I say from experience, you would be AMAZED at the kinds of special hits it can take.
Life orb Gengar cant ohko it with focus blast. Thats a LO boosted 120 power move coming from a Pokemon with 130 base special attack
doing 4x damage. He has those said weaknesses, but also a good resistance set including dragon, so hes a great switch in to specs dragons like Latios, which is made better by thunder wave, and most importantly, volt switch, which lets it switch in and shurg off a choiced special hit, and then gain advantage or momentum when the opponent brings in something to kill Probo, only to either have it paralyzed or have you volt switch as it comes out, damage it a little, and bring out a counter to their switch in.
Or plan B, just lead with it, pick thunder wave, take a hit, live with sturdy or just live), and then either volt switch out or setup stealth rock depending on the situation since youll go first.
Or plan C, give it protect because it has a stealth rock resistance, and one turn of leftovers will get it back to full health and sturdy) after switching in. That way you can take any hit, and thunder wave something that set up and was threatening your team.
^ You can run all of these things in one set.
He may not be fantastic but with a support set like that he will almost always be able to do SOMETHING beneficial in a match no matter what your facing, whether he just sets up stealth rock, checks dragons, saves your a** in a bad situation, or whatever. Thats not something a whole lot of Pokemon can claim.
~ inb4 Blissey is better in every way ~
Thank you captain obvious. Of course it is. Thats not the point though. The point is hes not as useless as you seem to think.
Also being an underdog like that is an advantage in itself. Everyone knows what Blissey can and cant take. Lots of people will try to hit Probopass with whatever super effective attack they have only to have it not die, and have themselves get paralyzed. Seems silly but its a valid thing to consider.