Here's my take on the whole thing... The phases are not "Monsters" so to say, they are more programs and were never meant to be battled by players, therefore they have infinite hp, now (correct me if I'm wrong) Though in the first battle with Corbinek, the Morganna Virus was put on to the weapons which caused "pain" to Corbinek, eventually forcing itself to put up the sheild. (Theories from here on out) Though this massive loss of power, due to the other phases demise and Bith's tinkering in the system, the plain storing the coma victims weakened and they became able to fight against Corbinek. Thus something dealing with Liminality the will of the coma victis and the dot hackers, the sheild broke. I honestly don't think the last "thing" you fight is Corbinek, but a last desperate measure of Morganna.
It looks nothing like the other phases and you don't fight it in the normal field where you fight the rest of the phases. Though it may not have been Morganna, I don't think it was a phase at all. Well that's my two cents,
EDIT: Well I found the functions of what the phases were...
Phase 1 Skeith: The Terror of Death
This program was designed to collect data on human emotions and reactions to death. The information collected by all Phases was copied to the ultimate AI.
Phase 2 Innis: The Mirage of Deceit
This program was designed to collect data on human reactions to deceit.
Phase 3 Magus: The Propagation
This program was designed to collect data on the human need to procreate and preserve the species.
Phase 4 Fidchell: The Prophet
This program was designed to collect data on the human concept of unavoidable fate. After the battle with this fourth phase, Morganna attempted to spread beyond the game servers to alleviate her increasing size which was causing heightened instability.
Phase 5 Gorre: The Schemer
This program was designed to collect data on human cruelty.
Phase 6 Macha: The Temptress
This program was designed to collect data on human affection (but only that occurs during unrequited love).
Phase 7 Tarvos: The Avenger
The program was designed to collect data on human jealously, revenge and other dark emotions.
Phase 8 Corbenik: The Rebirth
First Form: Attachments to Life
Second Form: Attraction of Death
Final Form: The Pure Heart
This was in the 4th liminality movie, whoops, hehe sorry everyone