Gruen is a character in Raider Zero.
Gruen killed Arkus Zei's parents, knowing of their riches, but didn't have the guts to slay Arkus, so he raised him as a pirate.
Gruen antagonises and taunts Arkus Zei as he explores the ship and eventually is fought as a boss midway through the Inner Quarters and The Rend.
On Easy difficulty; Stay on the platform by the entrance, at the back of it. Keep your shield up, and advance towards him. He'll back up and stop shooting for a few seconds. Shoot him with your current rifle when this happens. Once he resumes shooting, put your shield back up and retreat to the back of the platform again. Repeat until he is down to half HP.
On Medium or Hard difficulty; The same, except use grenades. They may be rare, but you'll need their damage.