­Wall of Denial + ((C28120))

Cheap shrouded wall of air; and a walking prison that would be utterly terrifying if you gave it regeneration or something. So hard to deal with by spells, and hard to deal with by creatures. (Actually, toughness 8 makes the wall quite hard to deal with by creatures, too. It's just a big stonking wall. At least proper shroud makes it hard to make nastier.)

So a non-vanilla creature that's hard to deal with in at least one way. are the colours remaining to me.

Ok; how about a creature that can't be pinged. Perfectly safe to stifle it, or imprison it, or whatever - but shooting it doesn't work. In fact, what the heck; let's make a simple "Shoot me! Shoot me!" creature. I know it's been done before; but I don't think quite like this - and it's pleasing.

((C28120)) is already terrifying, it doesn't need regeneration: it's extremely hard to kill in combat.

This is indeed a rather pleasing unShockable weenie. Particularly fun, as you say, if you have your own pingers; reminiscent of Sprouting Phytohydra that way, but the "noncombat" restriction allows this to be far cheaper.

Still not quite sure if this should be common - a burn spell being used on a 1/1 is infrequent enough that it mostly won't matter, and should perhaps be uncommon - but it is simple enough.

Has C28120 been deleted, then? I'm really curious as to what it was.
I like this guy.

Um. I guess so - and I'm not sure. A gelatinous cube, I think?

Maybe Gelatinous Cube*?