Marianna is the boss of Sewers level B.J. encounters in Wolfenstein RPG. She is the commander of Elite Guard and is in charge of the Castle Wolfenstein's defenses. She is also responsible for some experiments and mystical rituals that ultimately led to Harbinger of Doom summoning. Marianna is based on Marianna Blavatsky (and may even be the same person circa earlier in the timeline, assuming she recovered, see Id universe timeline).
In combat, she is aided by about 8 Elite Guards of different classes, plus a group of scientist, Prototype Super Soldier and an officer later arrive to assist her. The sniper rifle is the best choice to finish her off.