It took many, many attempts, and lots of dying very quickly, but I finally figured out the way to dodge Nightmare in his second “phase”. Well, I’d figured it out already, it was just because my Space Jump skillz were not ub3r1337 that I couldn’t put them into place. Some of it was also down to luck – namely, where he stopped. In certain places it meant I could fire off a million rockets into his face, in others only a couple.
Him down, and Gravity Suit GOT, I moved on a bit further – into an unexplored area of Sector 4 now.