It must be emphasized that he was not at his full strength, possessing merely one percent of his power from his prime. Otherwise, he would have already slaughtered everyone present. After getting involved, he even found it difficult to turn his massive body in such a confined space.
If Matins really went berserk with his power sword, he could stab him dozens of times in a short period, enough to kill him.
All he could do was to release more Plague Fly Swarms to harass the action squad while dealing with the interstellar warriors. Moreover, he would occasionally turn his head to "spit" at the squad.
His actions were effective.
In a short span of time, at least three elite soldiers met their demise.
One was surrounded by gigantic flies that failed to be cleared by firepower. Their sharp mouthparts pierced through the Exoskeleton Armor, injecting venom, leading to a swift death. The other two were hit by the spittle of the Great Unclean One and dissolved into a puddle of goo.