You only need a fastboot cable if you brick it in the process. I recommend this tutorial: http://forum.xda-developers.com/show....php?t=2100963
When you goto downgrade the b
ootloader please check the md5sum of the bootloader before flashing it. If you don't and its bad it will hard brick. Also a good idea to put the ROM and gapps on the sdcard ahead of time.
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adb shell "su -c 'reboot bootloader'"