Use a high level drake with lots of stam, (dont waste your time leveling up damage on your drake, it will be used for running) an ascendant shotgun with lots of shells, extra hazmat, and a buttload of Megalosaurus for actually fighting him. (The shotgun and drake are for fighting the tentacles)
Rockwell can be found in the Rockwell arena beyond his designated terminal, I believe it’s beyond the Graves of the lost (or commonly called the Rock drake nests) but it’s somewhere there.
Also to fight Gamma Rockwell like any DLC boss you need the three artifacts.
Stegos with 60+ armor saddles atleast 3 people at minimum shotguns 300-400 shots per person and 2-3 flak sets per person and 15-20 vegie cakes per stego walk around it and shotgun tentacles 200+ damage shotgun recommendation as its more effective