Greater Psi Reaver

The Greater Psi Reaver is a unique enemy in System Shock 2. It is encountered only during the fight with the Brain of the Many. Greater Psi Reaver has a huge number of Hit Points - as many as an Assault Robot. The projection regenerates 30 seconds after being killed, as long as its brain is still intact. It does attack often... and with thrice the damage of a regular Psi Reaver. A single projectile deals 60 hit points of damage to an unshielded player - the most damaging attack in the game. After we consider all of this, it becomes clear that it is the most dangerous creature encountered in the game.

Strategy
The strategy to defeat the Greater Psi Reaver is the same as with its lesser version: finding and destroying its brain before attacking the Greater Psi Reaver's body.

The Brain of the Many is one of the most difficult fights in the game, due to the large number of Rumblers and other Psi Reavers floating around. The Greater Psi Reaver is the icing on the cake, as the damage it does is enough to kill most characters in a single hit, especially on Hard and Impossible difficulties.

Your best bet for survival is to use any Agility-boosting items you have (such as Speed Booster), run to the small floor indentations that surround the Brain of the Many and smash Psi Reavers' brains kept inside them. The Greater Psi Reaver's brain will be among them.