John and Michael are walking back from dinner at around 20:00 when three men approach them brandishing knives, demanding that they give them their cell phones and money. As John was taking out his wallet, Michael notices a metal rod on the ground. He grabs it and hits one of the perpetrators on the head and the perpetrator suffers severe injuries. Michael is then charged with assault with intent to do grievous bodily harm.

Define "assault with intent to do grievous bodily harm". Discuss whether Michael has a defence or not.