The Final Battle Edit
As soon as Mercer says "Let's get this over with, shall we?", the fight ensues. Heller can use any power, but should he try to use the same power Mercer is currently wielding, he'll be immediately blocked and pushed by Mercer, with the addition of him saying things like "Copycat" or "Change it up, for God's sake!", thus encouraging him to use something else. It should be noted that it is useless to fight Mercer with bare fists, as he is invulnerable to them, even to the charged kick, the karate kick and the Bullet Dive (from Radnet). The battle can be divided into 3 stages.
"Shoulda kicked your ass days ago."
When Mercer is using the blade, it is recommended that Heller uses the Claws and the Hammerfists; for fast and powerful attacks, respectively. Claws do low to moderate damage, while Hammerfists can cause a fair good amount.
Mercer's attacks are deadly. It only takes one full combo to reduce Heller's health to almost half. Dodging is highly encouraged, and shields works just as well, depending on the situation. It should be noted that some of Mercer's attacks cannot be blocked, so the only way to survive them if Heller is low on health, is to evade them.
Mercer will use 3 types of noticeable attacks. The Blade Air Slice, Groundspikes, and Light Spear. The last one consists in Mercer tearing the ground with the blade (in a similar fashion of Heller's idle animation) and will go towards Heller at a very fast velocity. This is the only attack of the three that can be blocked with the shields.
When Heller successfully evades the Air Slice, he can perform either grab Mercer and jumping while doing a backflip, or violently smash him with the Hammerfists. Then as the fight continues, Mercer will use the Groundspikes 2 times. Heller can jump and Hammer Dive while Mercer is doing this.
Once Mercer's health becomes critical, he will run away from Heller and do a final Air Slice. Heller can evade it and smash him into the ground.
"Impressive. Now let's see how well you can adapt!"
Mercer gets up and he instantly regenerates all his wounds, making it seem like Heller's attacks did no effect. Then, he switches to Whipfist, grabs a nearby Blackwatch helicopter and throws it at Heller. The first one is easy to avoid. In this stage, Mercer's attacks are now ranged, and Heller will have to be very careful, as dodging is futile. However, Heller can use the shield to protect himself from his attacks, and leaving Mercer stun in the process, making him vulnerable. Blade and Tendrils are recommended. The Tornado Attack will inflict a heavy amount of damage to Mercer, while the Black Hole can immobilize him temporarily. Mercer will also attempt to grab Heller from a distance, and then Body surf on him if successful. Once his health becomes low again, Mercer will jump and try to grab Heller. Heller can shield up, and then do a painful slice to Mercer.
"Well now you've just pissed me off."
Mercer switches to his most powerful weapon, the Hammerfists, smashes his knuckles and then the ground. Then, he temporarily switches back to the Claws, to summon his loyal Flyers to attack Heller. However, Heller knows how to counter this. When the swarm gets close enough, Heller will explode, destroying all of the Flyers. Should be noted this is the only time Heller is able to do this, as Devastators (and Pack Leader) are unavailable during the fight.
This time, Mercer will go with all. Groundspikes, Flyers, Whipfist grabs, Light Spears, Hammerfist Cannonball and Hammertoss. In this stage, using the Shields will be the bane for Heller. Even with the shields up, Mercer can and will do a severe amount of damage to Heller if he is not quick enough to dodge. A single successful Hammertoss can be fatal if Heller doesn't have enough health. This time, using the Whipfist will be the best option. Keeping distance and attacking from range will be vital to win.
Once Heller does enough damage, Mercer will Hammertoss. Then, Heller evades it and with the Whipfist, grabs Mercer from the neck, jumps above and lands behind him, dragging him mercilessly to the ground.
The final round is entirely interactive, meaning there is no way of continuing unless the player successfully hits the buttons at the right time. If the player fails, Mercer will simply land a hit on Heller, and attack again until Heller dies.
Mercer is becoming impatient, and decides to attack Heller directly with a slightly different Air Slice. Heller blocks the attack, and slices off Mercer's arm with his Claw. He regenerates the damage and switches to Whipfist, where Heller grabs it, pulls Mercer forward, and cuts off that arm with his Blade. Then Mercer does a last effort with his Hammerfists, which Heller counters by blocking it with his own Hammerfists, and tears off both arms. Now, a defeated, armless Mercer lies on his knees. He cannot regenerate these wounds, as the damage is too severe. Heller grabs Mercer from the neck with his regular hands. Then, with a somewhat monotonous tone, Mercer says to Heller "Huh. Welcome to the top of the food chain.". Immediately, Heller changes to Claws and proceeds to furiously tear apart Mercer and finally consume him.
-All the movement, health, and regeneration upgrades should be at maximum to increase the possibilities of winning the battle. Mercer's attacks are strong enough to kill a no-health-upgrades James Heller in one combo.
-Since you are in combat, there is no way of regenerating health unless you have the last regeneration upgrade. It will be very slow, so try to win some time by dodging or jumping very high. Should be noted that regeneration is active at all times, including the interactive parts and stage transitions.
-Unlike other missions, there are no targets to consume during this battle, so keep an eye to your health bar.
-This battle has no checkpoints. In order to win, you will have to do your best to stay alive through the whole process.
-There is a way to further increase your chances of winning, however this method is only available through the Radnet Edition. One of the Radnet Mutation upgrades allows Heller to regenerate his health while his shields are up, by 1% per second. The regeneration stops whenever Heller is attacked. One good method is to jump and, when reached maximum height, activate the shields to have a break.
-When Mercer's health is depleted he will attack you with an specific sequence. Once you've successfully done the sequence, Mercer will stay still for a moment. You can seize this by not attacking, therefore gaining time to regenerate your wounds. Be alert though, as Mercer will also regenerate his own health (although not to 100% at least), and if you let too much time happen, the battle will continue.
-If you attempt to leave the area, Mercer will use Whipfist and drag you back, while doing a lot of damage as well in the process. Try not to jump too high or get away too much from him.