Mephisto will get an immediate extra turn when his Health is 0.
Any missions except Mission 1.12.5, Mephisto cannot be KO'ed in the traditional manner. If his health is reduced to 0 during the second wave (Spec Ops 4 - Mission 3 only), 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.
Also, if Mephisto is inflicted with either Blind Rage or
Depower, he will be unable to end the battle with Prince of Lies, thus causing the AI to loop endlessly, In the case of Blind rage, he'll eventually KO you after attacking you over and over, but in the case of Depower, you have to leave the game to end the fight.