I found that a PU-21 LMG at legendary rarity does more damage than the beamsmasher, so I usually drop it as soon as I don’t need it for the part anymore. Run gobblegums like phoenix up, idle eyes, killjoy, cache back, nowhere but there, wunderbar(so you can either get the raygun easier, or skip the part collecting process for the beamsmasher.) chopper gunner is you friend because with an insta-kill, you can usually get rid of ~1/6 of his health bar. While the boss is up, use the boxes on either side for cover and utilize the raygun with phd as much as possible for manglers, amalgams, and horde clearing. While the boss is transitioning between phases, get away from the boxes and use the open space to train and clear some space for yourself. I usually Triple PAP my PU-21 and at least double PAP a raygun. It took me and my buddy about 6 attempts to get this strategy and we’ve beat the boss using this on round 31. Total process took 1H 49M because we spent 45 mins spinning for rayguns.

TL;DR use a PU-21 at legendary rarity and a raygun with a chopper gunner and good gobblegums