I have been trying to get this achievement for ages, without luck. I've started several new worlds trying to get him to spawn. I did everything by the book. Wait until nightfall, went to the jungle and the underground jungle, pre-Hardmode, used a dragon summon and held a water candle so that the dragon killed enemies just as they appeared and the respawns kept coming at maximum speed.
No luck.
Eventually, I gave up and decided to open Hardmode on my most recent world. As I was trying to find Plantera for several hours, I figured to try cleansing the Crimson out of the underground jungle, as the bulb wouldn't spawn. After I cleared a good part of the biome, bam! I had 3 Doctors spawning in a row! I couldn't believe it! One of them spawned just as I cleared a part of the jungle with the Clentaminator, and two more after I left and returned to go on looking for the Plantera bulb.
That was Hardmode and no candle, so forget about those tips, they're useless. If you have difficulty getting him to spawn like I did, open Hardmore, let the corruption spread, then try to find a contaminated jungle and clear it during the night.