Fast AND efficient is the way to go, otherwise you just burn through a bunch of coins without any reward and get easily discouraged. There's a cooldown between uses that if ignored, will significantly decrease your chances. Waiting approximately 10 minutes between uses is the most efficient method that will net you the most "bang for your buck" or in this case, coins. Generally, the preferred method for exotic farming is to hang out in the Strike Playlists. Pick whichever you prefer (I like the level 36 one) and STAY IN THE LOBBY. You'll keep your Vanguard bonus and get more rep, which means more packages. You'll also get a shot at the strike specific gear which is never a bad thing to have drop. Pop a 3oC right before each boss fight (Flayers and Omnigul excluded) and then repeat. You should get an exotic from every 4-7 uses (on average). If you're just looking to burn through a bunch of 3oC as fast as possible, then the Regicide mission is where you'll want to go. Head to the part where the taken Ogre and taken Colossus spawn, kill one of them and then let the other kill you. You'll respawn right outside that room and will be able to proc the spawn of the two ultras mere moments after.