As most of the decks in the top 50 have no spell protection, and at most one miracle and one purify (more usually one or t'other), I ran this deck against them. Like the mono-fire, it's dull to play, and slow, but if you come up against the SSG (standard scaredgirl), it's essentially guaranteed victory.
Mark of Death
6xPoison
6xFirebolt
2-4xRain of Fire
Fahrenheit
Fiaahhh pillahhhz.
Most of the defences of the T50 rainbow decks consist of Sundial. So weapons and spells are the way I went here.
The main reason it works is the fast guaranteed damage of the poison, and the creature control/direct damage of fire bolts. You can definitely run this deck without rain of fire. Do NOT play Fahrenheit early, treat it as a one-shot damage card, because if you play fahrenheit early even the AI will dimly register that teh big shinee sawed might well be useful and nick it.
Also note: the AI does NOT know how to play the T50s deck well. You will be creamed if you try using this deck against a player. I don't put any creatures in, because creatures are essentially otyugh fodder.
Putting Deflag/Explosion in, or Immolation for faster damage tended to make the deck too complicated, so I avoided it. Ditto for upgraded Bone Wall - if you got it in the late game, it tended to get destroyed quickly, early in the game, it was too expensive even at just 5 death to bother playing. Arsenic has a tendency to get stolen. So...that's my Top50 deck. Have fun grinding those valuable quantum pillars - you may win an Otyugh if you're lucky.