why is legion just a badly designed DPS check?

[Brahm The seige Beast]

so rather than have a good boss fight with a cool gimmick its a buffed bs boss that has such high damage resist that i have to use meta sniper weapons to get past the damage reduction??? even when i hit this trash boss during a charge when his weakspot is revealed this trash just tanks. so. much. damage and for what?? i expected better than this not some stupid player dps/difficulty gate to slow me down so i take longer. who thought this was ok?????