Help! Bricked my Ardunio Mega2560 Rev 3

Please note that the adafruit usbtiny isp doesn't work with devices over 64K.
You will need to use a 'real' atmel avrispmkII, an avr dragon, or an avrjtagmkII.
There is a clone of the avrispmkII that does work, this clone uses an atmega32U2 or atmega32U4 device and is based on LUFA.