Arceus-Steel has some cool qualities: its immune to Toxic, can monoattack somewhat more effectively than Poison Arceus due Steel having no immunities, and resists a lot of useful types such as Dragon, Normal, Flying, Ice, Fairy, and Psychic. However, it struggles to make a name for itself due to Steel being somewhat lame as an offensive type and its common weaknesses to Fire and Fighting, making it hard to teambuild with. This team tries to support a Steelceus sweep by giving it opportunities to set up and putting tremendous pressure on its best checks.
Substitute is a great move on Steelceus. After one Calm Mind with these EVs, Specially Defensive Kyogre can only break the Substitute if it gets max damage with Scald, giving it very little chance of beating Steelceus by itself. Substitute also gives Choice Band Ho-Oh a harder time abusing Steelceus as you can just throw down a Sub to scout for the move its going to use and react accordingly. Other perks of Substitute include turning Ferrothorn into gigantic setup fodder, blocking Will-O-Wisp from other support Arceus formes, and stalling for SDef drops vs Mega Lucario if it wants to try and revenge you with Close Combat (in a pinch). Speed EVs outrun the aforementioned Mega Lucario btw.
The first thing that jumps out at you when you look at the teammates for Steelceus is that all of them can take on Ho-Oh and Heatran (Steelceus's best checks) in some way. This is important as it makes Steelceus much more difficult for the opponent to take advantage of. Gliscor immediately comes to mind when using Steelceus as its one of the better Ho-Oh switch-ins in the tier due to Poison Heal granting burn immunity. It's a very good Stealth Rock user, probably second only to Dialga, and can take on most Ho-Oh variants which is lovely. It is also invaluable for checking certain Fighting-types and most Ground-types. Gliscor also provides an Electric immunity to help check Zekrom.
Kyogre provides rain which helps mitigate Steelceus's Fire weakness, checks a lot of special threats such as Palkia minus Thunder, Yveltal, and most CM Arceus formes, and completely beats the crap out of Heatran. It can lure in setup targets for Steelceus such as Grass-types, particularly Ferrothorn. It also acts as a backup Xerneas check, mainly for Focus Blast sets since Steelceus can beat most other kinds (it can stand up to HP Fire if sun isn't out). Also works fine as a Kyogre check alongside Zekrom. This also tends to be the Wisp absorber.
One of the biggest perks of Steelceus is having a very sturdy Dragon/Ice/Fairy resistance, so that means Dragons become attractive teammates. Giratina-O is by far the best (probably only) Defog user to pair with Steelceus, and it does a ton of stuff on this team: it is one of two EKiller checks, my primary Fighting check, my backup Ground check, a Mewtwo check, and the Defog user. Giratina-O resists all of Steelceus's weaknesses while Steelceus resists most of Tina-O's, making them natural partners. Between Tina-O and Gliscor, I have the vast majority of Ground- and Fighting-types handled, and Fires are generally covered as well with Kyogre's assistance.
I needed some quick offense and a fast way to dispose of Kyogre and Ho-Oh so I stuck Zekrom on the team. Having a Flying-resist that can actually come in on Ho-Oh is nice, and it can create setup opportunities for Steelceus by Volt Switching on Fairies and Ferrothorn. It is also my second Mewtwo check which is extremely handy. Not much else to say, its a general purpose revenge killer and cleaner that works well with all the anti-Ground mon support.
The last slot is kind of odd. I wasn't comfortable with Tina-O alone against EKiller, I wanted some backup against Kangaskhan, but I also needed something that could quickly dispose of Ferrothorn without much trouble. Offensive checks to EKiller/Khan are very hard to come by, but Bulky MMX works well enough. It tanks +2 LO ESpeed after SR and KOes with Low Kick in the event Giratina-O can't handle it, and comes with Recover to make sure it can do that + switch into Ferrothorn. Even with no Attack investment, Low Kick still kills quite a few things at +1 such Palkia. MMX really really likes having a sturdy Fairy and Lugia check, so it works well with Steelceus. Zen Headbutt is needed so it can do something to Ghosts, and it does about 66% to Ho-Oh at +1 which will usually kill it combined with Brave Bird recoil, potentially opening up a sweep for Steelceus. In addition to Ferrothorn, MMX also gives a quick way to kill off guys like Dialga and Darkceus which is greatly appreciated as well.
You can potentially run Physically Defensive Yveltal > Mewtwo X, but I like the speed and offensive potential MMX brings to the table. Yveltal is much more solid an EKiller check and helps a ton vs MMY, but it cannot KO Ferrothorn quickly Heat Wave pls which sucks balls if it manages to get off a Leech Seed.