I would let some nut buster or equivalent soak at least over night, use heat, propane is usually not hot enough. Or take the knuckle out completely and support it on an anvil or vice as you give the ball joint stud a crack with the mini sledge. Maybe you could even support the knuckle with a jack stand, makes a huge difference with the pounding technique.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)