I see Roselia luring a lot of steels. My personal setup with her is a utility sweeper:I have 2 questions.
1)what is the best steel lure in OU?
Roselia @Life Orb/Black Sludge (Life Orb more often)
~Natural Cure~
Toxic Spikes
Sleep Powder
Leaf Storm
Shadow Ball
Timid
4 HP
252 SpAtk
252 Spe
So, since steel completely walls my Roselia (as well as any variant that isn't carrying HP Fire), steel switch-ins are common. Leaf Storm as well as the second attacking option, in my case Shadow Ball, and in other cases Sludge Bomb or Extrasensory, all are resisted by Steel.
So, the way to do it is to switch in Roselia on a weak type (Swampert always runs from Roselia unless they predict you well), drop Sleep Powder on the switch (added benefit of stopping Swampert if he counter-predicted your prediction of him switching and is trying to Ice Beam you), and start setting up Toxic Spikes. If you got yourself a Steel Type on the switch, you now have a sleeping opponent and Toxic Spikes up, and can switch in your Steel counter, I assume Magnezone. Even if Sleep Power misses or they wake up early you can bail and switch in your counter at any time.