In the comments on my Rust Monsters: Not For the Wuss of Heart post, several people claimed that they disliked Rust Monsters because they didn’t represent any real challenge: once you knew the trick of dealing with them, it was just a tedious process of beating them to death with non-metal weapons. I think that betrays a lack of imagination on where and when they might meet Rust Monsters, but for them I have created:
Dire Rust Monster
Armor Class: 2
Hit Dice: 5*
Move: 120′ (40′)
Attacks: 2 Claws/1 Club Tail/1 pair antenna
Damage: 1-8/1-8/1-8/special
No. Appearing: 1-4 (1-4)
Save As: Fighter: 3
Morale: 7
Treasure Type: Nil
Alignment: Neutral
XP Value: 400
Looking more like an ankylosaur with antenna than an armadillo, the Dire Rust Monster share with its lesser cousin a voracious appetite for metal.
The special antenna attack is the same as a Rust Monster: non-magical metal armor or weapons crumble to dust; magical metal armor or weapons lose a plus (10% per plus chance of resisting the effect), once it loses all its pluses the next hit crumbles it. A successful hit on the monster with any type of weapon means the body was hit, and there is no ill-effect on the weapon.
Now it’s just as challenging as an Owl Bear (since it’s statted like an Owl Bear, except for its AC, a lower Morale and a different special ability, and a whopping XP bonus) and those who were bored by the original Rust Monster should be all eager to go up against it, right?
Tales of the Rambling Bumblers 
Oh, whenever we met rust monsters, we knew there was an owlbear around the corner waiting for us to strip off our armor— that’s part of why it was such a DM cheat.