With Relentless Pony, I did, all in the same game...
-The original Relentless Pony combo (give Pony haste, attack, bring it back from the dead, give it haste, attack, and so on).
-Mind over Matter a player's own Mesmeric Orb (although I think I also saved him from being decked).
-Play 15 copies of Snowfall, Opalescence, give them haste, attack.
-Kill with Leeches damage.
-Demonic Attorney (actually I think it was a different ante card, but whatever).
-Meditate away a bunch of my own turns and let the surviving players kill each other.
In another game...
-Donate a Sliver Queen to one player, use MoMa to untap his lands and let him generate some number of tokens (Pheldagriff would be easier for this under the new rules I guess?). Use a lifegain card to give the other player a ton of life. Concede. Not technically a kill, but it was fun to see them try to work around each other's advantages.
Ones that I didn't actually ever do in a real game, but that would be awesome...
-Use various effects to put all of opponent's cards into his hand. Echo Chamber in a ton of token copies of Tempest Efreet (using Donate, as I did in the original engine with Ring of Ma'Ruf). Force the opponent to kill choose between conceding, killing himself with life payments, or trading me his deck for a bunch of tokens.
-Mindslaver chain reaction. This might not even require a Relentless Pony engine (by which I really mean an infinite generation of Ring of Ma'Ruf tokens off Echo Chamber and a Donated Ring), but that's the easiest way I can think of. Use Mindslaver on everyone and then put Mindslaver under their control and use MoMa to keep untapping their lands for mana and repeated uses of Mindslaver on some other player. I can elaborate if this seems confusing, but the idea is to make it so that for the rest of the game, Player A controls all of Player B's turns, Player B controls Player C's, etc. The only reason I never did it was that I took Relentless Pony apart before I had a Mindslaver in my collection.
-Shahrazad recursion (but I don't have a Shahrazad).
