Ok i've read this novel now, my review for it is up in the new BL Reviews Section, and i'd like to set the record straight on that Lexicanum article because it's quite inaccurate.
As to the first fight it is not anything like the article describes. Legienstrasse does not fight all of those enemies, she only fights the Grand Mistress of the Culexes Temple. She wins obviously because a Culexes Assassin is a psyker-killer and isn't able to deal with mundane threats with the same level of efficiency, and she was unprepared for how strong Legienstrasse actually was. The Vindicare Assassin, who is NOT the Grand Master, fires a single shot to help out but Legienstrasse knows how Vindicare operate so she was able to predict the most likely spot he would pick and the most likely time he would fire, and block the shot. After that the Assassin couldn't continue firing because the Culexes was in the way. The Imperial Fists were fighting other enemies and only Lysander was trying to get to the fight, and Legienstrasse used tentacles of solid blood to hold him back. Only an Assault Sergeant made it to the fight and he was quickly outmatched, and Legienstrasse used his biomass to end the fight with the Culexes quickly. She fled after that.
And in the second fight once again she doesn't actually fight all of that. She fights Lysander, two of his Terminators, the Vindicare and an Eversor and the Thunderhawk, and not all at once. She fights Lysander and is at a disadvantage until the two Terminators come in, she kills them and uses their biomass to fight Lysander. They are fighting in an unstable warp-built structure and they fall into a cavern of magma, where the Thunderhawk shows up with the Vindicare on board. He fires at her and tries to support Lysander but Legienstrasse assaults the Thunderhawk, killing the Vindicare quickly and rips the Thunderhawks' left wing off, causing it to crash. She gets wounded by the damage and that gives Lysander enough time to wake the Eversor up who puts up a good fight but can't match Legienstrasse's morphing abilities. He loses and just before he dies Lysander uses the fallen Emperor's Champion's sword, who had died fighting a World Eater Berzerker Champion, to pin them together. The explosion wounds Legienstrasse and allows Lysander to decapitate her and hurl her head into the lava. And with that the Maerorus are extinct.
That article is dead wrong in the details of the fights. Legienstrasse never fights that many enemies at once, and in every fight she had the advantage of being surrounded by dead bodies to use as biomass. In her second fight with Lysander she actually didn't morph beyond turning her hands into bone blade until the Terminators came in and provided biomass, that was her key weakness. Without bodies to absorb she couldn't utilize her power, so a significantly capable opponent could prevent her from absorbing bodies long enough to kill her.
Just wanted to set the record straight.