Play on Nightmare Difficulty on a map with Survival Mode enabled (you must complete the map first on Nightmare to unlock Survival Mode. Reaching at least Wave 15 on Survival mode gives you a chance to randomly get Transcendent weapons from drops, chests or NPCs on the map. Higher waves drop more Transcendent gear each wave. Note that you only get maybe 0-2 Trans items per wave from Waves 15-20ish, and after that only about 5-10 if you're lucky. They're very rare.