The Marauder is fucking terrible precisely because he breaks up the combat loop by regressing the flexibility and openness of the game systems to singular combos or functions that must be conquered in order to progress. It's the same with the purple sludge before you get the super shotgun; you're stuck in the sludge moving slowly, can't dash can't jump, until you can find away out. It functionally does nothing interesting at all, except take away game systems. The argument is this in itself provides challenge, which is true, but is a poor way to do so in my opinion. Similarly for the Marauder. His combo loop isn't hard, his window is easy to predict, but he's built with such an obnoxious set of strict engagement requirements that fighting him is unlike fighting any other enemy in the game. You have to play the fight on his terms, you have to space yourself correctly, and you have to use the combo window or exploit splash damage to beat him. He exists for no reason other than to push the player in the middle of encounters, and he alone is not fun to fight due to the boring, rigid simplicity of his formula. So in the end he is frustrating at worst for how he disorientates the player with his incredible speed, strength, health, damage, and everything else, and boring at best as when left alone it's repeated one-two-punch combo repeated over and over and over until he's dead.
Regressing game systems for challenge is not fun. I complained earlier that the combat felt rigid, and I was advised that it feels less so as you get more gear and power up. This advice was absolutely right. Marauder is the antithesis to this philosophy by explicitly regressing the combat system. It's like if Gracious and Glorious didn't just take away witch time, but could only be damaged with one specific combo and none of the others. It'd make their encounters super fucking boring and rigid. That's the Marauder. Should have been cut.