Heller gets one in the tutorial, when a Brawler jumps him. This is a beast capable of tearing apart tanks and can casually bite a human head off. Heller, pinned beneath this thing and armed only with a knife, kills it with a combination of fury and repeated stabs to the head and neck. There's a reason why Mercer was impressed enough to convert Heller without any further credentials.
Before that, Heller sees Mercer landing right in front of him. Disregarding Red Crown's orders to wait for reinforcements, Heller attacks Mercer with only a combat knife, and manages to get a few stabs in, even though Mercer is obviously unaffected and unfazed and returns the favor shortly after, complete with a Kung-Fu Sonic Boom strike that sends Heller flying. And yet, as shown above, and he stillgets back up from that. There's a reason why Mercer chose to test Heller instead of killing him then and there.
The penultimate mission counts as well. Heller's daughter is taken hostage by Rooks in the GENTEK building behind an impenetrable gate that even Heller's Hammerfists can barely dent. What does Heller do? Consume the nearby Alpha Juggernaut, granting him the ability to summon other Juggernauts, and orders them to smash the gate for him while he demolishes the attack force with his One-Hit Kill powers granted by the Alpha he absorbed.
In the preceding fight, if you do the dodges even halfway right, you come off as if (and Heller explicitly notes) that you're running rings around your opponent. Heller's so fast that Mercer can't even keep up with him.
Heller: I'm too fast for you, asshole!
In fact, even Mercer gives you a rocking round of applause (metaphorically, given his arms were ripped off) for decimating him.
Props must be given to Mercer. If Heller attacks him using the exact same power as he does during each stage of the boss fight, he will easily counter Heller's attack and deal a nasty blow, all while taunting him, showing that Mercer had the clear advantage in experience compared to Heller, prompting Heller to take a different approach, while Mercer continues to switch between his powers.
The fact that Mercer's Hammerfists can bypass Heller's nigh-impenetrable dual-shields. Remember that those shields are able to deflect tank shells and hellfire missiles, and the only other attack that is able to bypass the shields is from a Goliath.
Heller's victory becomes even greater when the final battle of the first game is taken into account. Mercer has already cut down a Superior Successor, the Supreme Hunter, so he is prepared for Heller. Heller's sheer tenacity and cleverness are just that good.
After seeing that he's outmatched against Mercer in terms of experience with the various powers, Heller takes a different approach and counters Mercer's appendage with another. Mercer uses his Blade? He uses his Hammerfists. Mercer changes into his Whipfist? He changes into his Blade. Mercer uses his Hammerfists? Heller responds with his Whipfist. It basically shows how well Heller can adapt.
In terms of gameplay, Prototype 2 blows Prototype out of the water. Now you have shields on both arms and you can perform shield bashes, which means you can leave enemies open for a devastating attack. Sometimes it's slicing off their arms, in the case of Brawlers, sometimes it's giving them a German Suplex from fifteen feet high, in the case of Orions. You can consume people in the air now, by slamming them back first into the pavement and crushing them, bouncing back up to continue gliding.
From the "Excessive Force" DLC Pack...Two words: Medusa's. Wrath.
After consuming a scientist in the Red Zone and getting into a helicopter while disguised as the Doctor, an Evolved comes and attacks the helicopter, demanding to know where the scientist is. Heller gladly responds.
Heller: The Doctor is out, motherfucker! *kicks Evolved out of the Helicopter*
Step 1: equip the claws. Step 2: target a helicopter. Step 3: use the pounce move, and feel like Wolverine as you tear it to shreds. Bonus points if you use the claw finisher.
The finisher moves against vehicles. If it's a tank, Heller rips the turret off it and swaps the hull with it. If it's a helicopter, with the proper powers, Heller could either uppercut the hull, claw out its rotor, whip the tail rotor off, or even split it in two!
In the opening, Mercer karate kicks a helicopter. Ascended Meme, anyone?
See a blue barrel or a white container in a Gentek facility? Punch it, sit back, and enjoy as all the mooks and civilians in the area become infected with Whitelight and goes to attack and infect other people. Yes, this game allows you to start your own Zombie Apocalypse.