From the link
"Additionally, the eudaemon segment used in reconstructing the eudaemon key item affects what kinds of special effects are present on each level. Vanquishing particular NMs within each section will remove these effects."
And "Special foes may be present in an Alluvion Skirmish depending on the type of eudaemon segment used and the area where the battles take place. These monsters confer special benefits and weaknesses on all enemies appearing within the Skirmish, including bosses. These effects are naturally double-edged swords, and which monsters confer which effects can be found in the table below. These monsters can see through all attempts to avoid them and will link with nearby monsters, so take care should you choose to leave them alive."
tl;dr certain parts will give a buff AND and nerf to all mobs including the boss in the zone and provide you with a special mob that if killed remove those effects so you might want to keep that mob alive
XI Wiki


