The galaxy nexus is very hard to brick..
1. Download a custom recovery from the clockworkmod site.
2. Boot into fastboot and type: fastboot flash recovery the-name-of-the-recovery-you-just-downloaded.img (for this to work you need to paste the downloaded recovery.img into the fastboot folder)
3. Boot into recovery and select "install zip from sd card" (assuming you already downloaded the lastest supersu.zip)
4. Flash the latest supersu and wipe both cache and dalvik cache.
5. Reboot and enjoy.
Sent from my Galaxy Nexus using xda app-developers app