Proteus was my second 4* ever, so I got used to bringing him on almost all my raids. I also pulled Sabina early, and by that time I had 2 of every other color except Yellow. So I would often run a 2-2-1 stack in Raids, with Proteus and Sabina in almost all of them (even against Purple tanks sometimes). I now have a better bench and usually do 3-2 stacks, but Proteus and Sabina are almost always in my attack team still.
It was NOT easy against those Purples, and I still curse Cyprian or Rigard to this day
But Proteus and Sabina both have great general utility. For Proteus, just remember that cleansers (Rigard, Vivica, Zim) can cure his mana stop.
I understand why you want to bring Gormek, Sabina and Wu always. They’re you’re maxed 4*s. But if you bring Proteus as a 4th, then that leaves only Gormek as a direct damage dealer. However, with Wu and Gormek, that means you’re aiming to kill the enemy with tile damage. Proteus helps there; once his mana stop is in effect, you can dump as many tiles as you want into enemies without them gaining mana and firing off your specials. Boldtusk would be generally good here, the attack buff adds to the effectiveness of the “kill em with tiles” strategy.
But Greywing gives great advice. There is no “one size fits all” for building an attack team. Choose your team based on the defense. Try to bring 2 heroes strong against the tank color, and 2 more strong against the flank. But it also depends. If your enemy has a riposter for example, you want a dispeller - even if your only dispeller isn’t of a strong color.
Still, you can’t go wrong using Proteus, especially if you max him up