Just to give you some background. I have two other people that i regularly play W:I with. Every now and then one of us will come up with a deck that is unbeatable  until one of us figured out how to beat it. 

One of the players has made a chaos deck that hasn't been beat yet and he;s probably used it for close to ten games. My newest theory is if i can make him discard cards from his deck and hand he will lose those plague bombs and insta/perma- corrupts. 

I'm having a hard time finding cards that make your opponent do this. Please list any cards you know of that work like this, include scouts as well.

thanks in advance.

Deck Discard

Dark Elf

• Bathe in the Cauldron (Tactic) - Discard the top 5 cards of deck.
• Witchbrew (Support) - Attach to unit, when you attack discard the top card of player's deck
• Witch Hag - Corrupt to discard top card of deck
• Frenzied Witch Elf - When attacks discard the top 2 cards of deck

High Elf

• Caradryan

Neutral

• The Wild Hunt (Tactic) - Not a discard card, but will shrink deck
• Infiltrate (Quest)
• Dark Abyss (Support)
• Entropy (Tactic)
• Plague Monk - Needs a Spell to Fire

Hand Discard

Dark Elf

• Bladewind (tactic)
• Caught the scent (tactic)
• Anointed Cauldron (Support - building)
• Harpies - This only works when opponent has 7+ cards in his hand
• Vanguard of Woe
• Beastlord Rakarth - Doesn't discard, but removes a card from hand nonetheless

Chaos

• Will of Tzeentch - Each player discards his hand and draws 3 cards

Neutral

• Warhawk Rider - When this unit attacks or defends, target opponent discards a card.

I've probably missed a few, but that's the main ones I could find/know of

 Scout keyword

• Dwarf Ranger
• Free Company
• Gutter Runners
• Shades
• The Changeling

 thank you very much! It seems like the dark elves would be the best choice. 

Yes...the Dark Elves are definitely the best choice.

Yeah. Dark Elves are THE choice. A friend of mine has a deck with Dark Visions (Allows you to choose 1 of top 5 cards to add to your hand), Shades and Rakarth. So what happens is that he on average he has Rakarth + Shades on a second-third turn and makes me discard two cards per turn, which is super bad.=)

PS. Does anyone use a Wild Hunt? It seems to be a very tricky card to play (After all, the controller of a sacrificed unit may chose whether to put top X cards of his deck as developments or may chose not to..=/ )

Discard is good only with Dark Elves. I recommend Plague Monk and shit-load of cheap DE spells (Dark Visions, Chilwind, Bladewind to name few) supported with Frenzied Witch Elf + Witchbrew, Witch Hag, Bathe in Cauldron.

Shades are nice as well but i prefer to spend 2 resources on playing two spell and discrading 4 cards from opponents deck thanks to Mr. Plague Monk.