There's one way to get past an invulnerable mob: it takes damage from super attacks (its shield can be seen by the diminishing white bar above its head). It took me two supers to kill it, but unfortunately, my character build wasn't optimized for supers, and I wasted a ton of time. I soloed it with a Void Titan (axes) and encountered two invulnerable mobs in one playthrough, but luckily the first one was chasing me during the boss fight, so I didn't have to kill it. P.S.: I won't be returning to this activity because of this bug, as it's not a normal situation, but my "life hack" might be useful for those who want to get weekly tokens.