Basically, the things that are real in the universe the character comes from will perform like a real creature. When damaged, they'll bleed, go into shock and slowly die the way a real living thing does. Their attacks will deal physical damage to the player's body and inflict bleed. When they die, their body remains there and performs like a regular object. This makes these fights seem quite real.
However, the things that are completely fabricated by the player character will not perform like real creatures. When damaged, they just lose a fixed amount of health instead of bleeding. They don't go into shock, and keep functioning perfectly (aside from body damage, they still feel that) right up to the point of death. Their attacks don't deal physical damage, instead dealing fake damage composed of two other status effects (fatigue and pain) that mimics the effects of real damage on the player's body only semi-convincingly, and fake bleed that takes from the stamina and will meters instead of health and thus only mimics the actual effects of bleeding only semi-convincingly. These effects also all go away much faster than real damage heals, and the visual effects of the damage they're supposedly dealing will go away faster still instead of lingering until the damage heals the way regular wound graphics would. Finally, something strange (but different for each one) will happen to the bodies of these creatures that removes them from the game. Dissolving, exploding, catching fire, fading out of existence, vanishing in a flash of light, launching into the sky to never return, so on. Some of these effects (like them catching fire or exploding) will be able to harm the player, although of course with fake damage. This all makes the fights much more surreal, and much less believable.
So, how's that? Is it a good enough distinction that the player can pick up on it when they're not told they're in a psychological landscape, much less that some of the enemies are based on real things their character has encountered and some aren't? Because the distinction needs to be that strong.