Few can claim to match the evil might of the arch-demon Mephisto. His power in our dimension is small as yet, but grows stronger as he acquires the souls of mortals. To do so, he must subjugate another's will through the victim signing a demonic pact.
In Spec Ops 4, Mephisto cannot be defeated in the traditional manner. If his health is reduced to 0 during the second wave, he will regain a small amount of health relative to your agent's level. The battle can only end when he uses "Prince of Lies." It is advisable however that your agent along with Ghost Rider (Team-up Hero) and the other hero you use must be kept alive in the second wave. This increases the probability that Mephisto will trigger his 'Prince of Lies' ability, ultimately ending the battle. Mere one-on-one fights with Mephisto (e.g. Torch vs. Mephisto or Agent vs. Mephisto) may not trigger the 'Prince of Lies' ability soon enough, thus prolonging the boss battle.