Poison Jab or Brick Break. Those both require breeding so if you don't want to do that, you just gotta wait until they are available.
Breeding before Episode 13.
I would say Ampharos. As you said you already have steel covered. Amphy offers a wide array or coverage moves
And who knows? Maybe we will be able to mega evolve it?
Dragalge is a really good Poison/Dragon Type
Metagross is as you probably know, very good.
Mamoswine is a noice ground type.
Ampharos is pretty chill
I can't recommend Roserade/Tangrowth enough
That's my two sense at the moment.