actually Nexus 7 is so resistant to hard brick in my experience.
It is so easy to unlock bootloader.
1- install adb drivers and having fastboot application on your PC
2- enter to bootloader (turn off tablet > hold Vol - > power on)
3- connect Nexus 7 via USB
4- run in command prompt:
fastboot oem unlock
this will ask you to confirm factory reset and wipe all your data and voiding warrenty
after this, you have to flash a custom recovery (TWRP I prefer) to flash a custom ROM (I myself use Dirty Unicorn) and so on.