UAC R&D invisible enemies
Once you've defeated the spider demon, go through the door that was directly behind it, and you'll find yourself in the final room of the level, talking to the scientist. After you talk to him he will activate the portal and send you to Gehenna. Immediately jump back into the portal, returning to UAC R&D, the first demon you confront, lead it back to the portal room, when the enemy enters the room, it will be invisible..this will work with most enemies..
This isn't a trick, but don't equip armor shards, convert all shards to flak jackets. This will significantly increase your defense.