download TWRP image then go to download folder
adb reboot bootloader fastboot devices # just for check
fastboot flash recovery twrp_3.0.2_mate7_6.0.img
fastboot reboot
download zip file of SuperSu and put it on root folder of Phone SD card
adb reboot recovery
install zip
References
http://forum.supersu.com/topic/908/huawei-phone-general-guide-huawei-root
http://www.droidviews.com/how-to-boot-android-devices-in-fastboot-download-bootloader-or-recovery-mode/
https://android.gadgethacks.com/how-to/know-your-android-tools-what-is-fastboot-do-you-use-it-0155640/