Phase 1: Mind Games
Psycho Mantis is a psychic. He will read your memory card and comment on other Konami games you've played. He will also predict your movements, making him almost impossible to hit. To defeat him, you must switch your controller port. Unplug your controller from Port 1 and plug it into Port 2. This breaks his psychic link. He will also use telekinesis to throw objects at you; shoot them down with your SOCOM or FAMAS. He can also possess Meryl; punch her to snap her out of it.
Phase 2: Vulnerable Psychic
Once you've switched controller ports, Mantis can no longer predict your moves. He will still use telekinesis and fire energy blasts, but you can now hit him. Use your SOCOM or FAMAS and aim for his head. Keep moving to avoid his attacks. He will occasionally turn invisible; use your Thermal Goggles to see him. This fight is more about understanding the unique mechanic than raw combat skill. Reward: Card Key Lv. 5.