Your chances are greatly increased by getting scores over 110K in Nightfalls, Prestige Nightfalls well help you customize the score multiplier. I've gotten two catalysts (wardcliff and merciless) this way in less than 10 nightfalls, but I've also gone over 40 without getting another yet. Not every one is so lucky because it's still rare, but Bungie confirmed by replying to a thread about this during the DFA farming week saying that high scores are the best chance you have at catalysts and specific rewards. For Modifiers, I recommend Match Game (get appropriate burns for enemy shields in the nightfall), Famine, and either Heavy Weight or Extinguish. Tune your handicap lower to raise the multiplier to about 3.0x, and most strikes will be able to hit 110K+ score. You'll have to decide which burn you want depending on how you want to fight the enemies and the boss.