Downgrading should not put you into any jeopardy of bricking! It' s all done with the stock recovery and a method build into the stock rom - what did you try?
Anyway - you download the .8 firmware for your SKU or region (US, WW, JP)
- you unzip that file once, the result being another zip
- you copy that file to the root of your internal storage
- you should get a notification that the downgrade is available. If you don't: reboot
If that does not work try method 2:
- rename the unzipped zip to EP201_768_SDUPDATE.zip (make sure your PC shows file extensions - you don't want to end up with EP201_768_SDUPDATE.zip.zip)
- load that file into the root of a fat32 formatted microSD, insert the card and boot to recovery
- the recovery should recognize the firmware file and flash it automatically
To boot to recovery you boot up holding Power and Volume Down buttons until you feel the tablet vibrate twice and see tiny script appear. You'll also see three icons: RCK, Android, Wipe Data
RCK is flashing. Push the Volume Up button to enter recovery
Once that is done you download the Motochopper tool and follow the instructions to root the tablet.
Ignore everything but the numbered instructions in the grey box. It doesn't say so, but the tool works for the TF700 stock JB rom. Naturally you need the Asus drivers for your device - not Samsung as it says there. If you can connect to your tablet in MTP through USB you should be ok.