Schumann had held the teenager in a headlock while another man attacked him while at a cab rank, only to deal his own punch to Mr Merz’s head after releasing him.
Mr Merz was knocked unconscious and suffered convulsions. He was released from hospital with facial injuries and is in need of ongoing medical care.
Schumann pleaded guilty to a charge of assault occasioning grievous bodily harm.
His lawyer Natasha Bates told the court Schumann had a history of violent behaviour after being abused as a child by his father.
She said Schumann was hit first on the night of the attack by a friend of Mr Merz, but not all of that incident had been caught on camera.
“Given the context in which this assault occurs, it should not be characterised as gratuitous violence,” Ms Bates said.
Schumann has also be ordered to pay $2,000 in compensation and has been banned from Fortitude Valley for 18 months.
The attack on Mr Merz occurred one day after 1,000 mourners farewelled one-punch victim Cole Miller.
The 18-year-old died after suffering massive brain trauma from an attack on January 3, also in the Valley.