In B/W, unfortunately it is not possible to hunt it before post-game. Darumaka does not spawn in any Hidden Grottoes, and only appears in normal sand encounters, which do not give an HA chance. And because Dream World is shut down, the only way to get one before post-game is to be traded one, or traded a Zen Mode Darmanitan that you could breed on Route 3.
However, in Black 2/White 2, it is possible to get a HA Darmanitan before post-game.
If you have a copy of BW, (which I assume you do because of the question you asked,) you can connect to your B2W2 via Memory Link. Once you enter Driftveil City and receive N's Zorua from one of the sages, you can see a cutscene in Memory Link that activates the N's Pokemon quest. Once you see this cutscene, you can go to Desert Resort and find a Zen Mode Darmanitan in the sand as a normal encounter. If you catch it, you could trade it over to BW, where you can breed it to get a HA Darumaka.
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Hope I could help!
