Other tactics weren't working for me. Sabbia Vivente does well enough for me, as it has a couple of turns where it is faster than Bruce, which I found made things easier. This somewhat limits the AoE damage, as well as the healing the sharks can have when killing off your pets.
Sabbia Vivente having Tempesta di Sabbia helps mitigate some damage and by being ahead of Bruce it generally makes it somewhat more reliable.
Dardo di Sabbia helps against Maws Jr. as well as it means Sabbia Vivente is faster and is able to dish out some extra damage.
There's still RNG involved here, it seems that Bruce seems to go through patterns where he uses his attacks in a certain order for a couple of fights before he switches it.
Mastino Ardente shouldn't have too much difficulty against Maws Jr. Most successful runs Maws Jr. was already down to 3/4th if not half of his health.
I am going somewhat by memory here, I'll try to check if there's any major issues when he's next up.