Setup: You have 4 cards in hand (as if P1). Opponent has 5 cards in hand (as if P2, so one is coin). 26 cards remaining in your deck. 10 mana crystals. The board is clear. Both you and your opponent have 30 health.
Goal: This turn, kill yourself using only fatigue damage.
Rules: You may use any legal constructed deck, you may pick a legal constructed deck for your opponent, and you may assume perfect shuffling and RNG luck. Assume this is the first turn in every way except that you have 10 mana crystals to work with. (For example, you can't assume your opponent played Millhouse Manastorm.)
A possible solution:
Play Priest vs Mage.
Opening hand: Mind Vision x2, Thoughtsteal, Mindgames.
Play Mind Vision, receive Sorcerer's Apprentice. (Mana 1/10, drawn 4.)
Play Apprentice. (Mana 3/10.)
Play Thoughtsteal, receive Apprentice x2. (Mana 5/10.)
Play Apprentice x2. (Mana 9/10.)
Play Mindgames, putting Gadgetzan Auctioneer on the board. (Mana 10/10.)
Play Mind Vision, receiving Coin (from opponent), drawing Mindgames (from Auctioneer). (Mana 10/10, drawn 5.)
Play Coin, drawing Thoughtsteal. (Mana 10/11, drawn 6)
Play Mindgames, putting another Auctioneer on the board and drawing some card. (Mana 11/11, drawn 7.)
Play Thoughtsteal, receiving Arcane Intellect x2 and drawing two cards. (Mana 11/11, drawn 9.)
State at this point: three Sorcerer's Apprentices and two Gadgetzan Auctioneers on the board. All mana used. Two Arcane Intellects and three arbitrary cards in hand. 9 cards drawn from our deck (so 21 remain).
The minimum amount of overdraw necessary to die from fatigue is 8 cards, which does 1+2+3+4+5+6+7+8 = 36 damage. So we need to draw a total of 29 additional cards on top of those we've already drawn, using only spells with mana cost at most 3.
Cast both Arcane Intellects to draw 2*(2+2) = 8 more cards, so we need 21 more draws. Cast two Power Word: Shields to draw 2*(1+2) = 6 more cards, so now we need 15 more draws.
It's enough to find 4 Priest spells with mana cost at most 3 that we haven't used already, because by casting each spell type twice, we'd get 8 casts to draw 16 cards. In fact we can find 8:
- Circle of Healing
- Silence -- note: if used, use these last, on a Sorcerer's Apprentice
- Holy Smite -- use these on opponent's face
- Inner Fire
- Divine Spirit
- Mind Blast
- Shadow Word: Pain -- note: if used, use these second last (before silence), on Sorcerer's Apprentices
- Shadowform
Edit: Fixed bug noted by EgorK. (The fix makes the solution a little stronger.) I'm interested to know whether any substantially different solution is possible other than the one above.