When the White Cyclops takes Celica, a new status bar appears: Mind Control. When the mind control is full, only then will he use Celica's body to open the portal.
I'm facing the white cyclopus alone and I kill the other two cyclopus, I don't use any glace/violet armor, before going to fight I prepare 1 anti-sleep potion, 1 anti-bruise potion, 1 magic potion, 1 regeneration potion, all of these at 40%, but I just leave these potions to fight the final boss (which in this case never appears)