Spray the bottom bracket with a penetrating oil and let sit then repeat and lightly band with a hammer, this will allow the oil to seep into the threads and break the rust. Try getting a piece of pipe about 3 feet long that will fit over the handle fo the adjustable wrench. This combined with the length of the wrench will you enough leverage to turn the bottom bracket.
Word of advice; don't put too much force in the beginning as this could lead you to strip the threads. Do a gentle bouncing up and down motion until you feel it give.