Enchantments like Homicidal Seclusion (which gives a solitary creature +3/+1 and lifelink) or Predator's Gambit (which gives a solitary creature +2/+1 and intimidate) can make any creature terrifying enough to rip your opponent apart. And once you have a creature with immense power, take advantage of it with Triumph of Cruelty and Essence Harvest!
Playing with just one creature can be risky since you leave yourself open to counterattacks, but the deck has a number of ways to fend off your opponent. Frost Breath and Crippling Chill lock down opposing creatures. Into the Void and Peel from Reality return creatures to their owners' hands. (This is also helpful for getting you down to a single creature!) Alchemist's Apprentice and Crypt Creeper are especially useful minions-not only can they block potentially painful attacks, but they can sacrifice themselves at a moment's notice when you'd benefit by having just a single creature.
If you're interested in customizing this deck, try adding Descend into Madness, an enchantment that will devastate your opponent's side of the board while supporting your solitary strategy. Another strong choice is Tamiyo, the Moon Sage, a Planeswalker who can distract your opponent's creatures and refill your hand with cards.
