Time for the final showdown. When you're ready, head for ground zero, the place where it all began at the southern end of the Historic District. Kessler will meet you and you'll have to fight him. This is your toughest fight, but it's mostly an exercise in button mashing and constant movement.
Your main weapon is going to be your Lightning Bolt. There's a specific instance when you can unload some Hammers or Grenades on Kessler, but for the most part, using any special Powers is a waste of energy.
Kessler[edit]
Kessler can teleport extremely quickly and he has a variety of attacks.
- Grenades
These are similar to your own Grenades, but with a longer fuse. Provided you don't actually get hit by these, they're fairly easy to avoid. Kessler doesn't throw them far from his body, and while he's throwing them, he stands still, allowing you to hit him with some Lightning Bolts. You can also try to shoot the grenades, which will probably start a chain reaction, blowing all of them up. Don't be close to them when this happens.
- Sonic Punch
Kessler starts teleporting around, and suddenly appears in front of you, winds up and delivers a punch enhanced with a shockwave. You can roll out of this. When he starts teleporting around, pause and wait for him to appear and wind up, then dodge to the side. You should be able to avoid most of the effect.
- Earthquake
Kessler pounds the ground and a shockwave radiates through the ground. If you get caught in the blast, you'll be knocked down and take a lot of damage. Once Kessler gets down to 50% health, he will add phantasms to the effect. You can avoid the effect altogether by jumping and using Thrusters to float over the wave as it passes by you. When the phantasms show up, pepper them with Lightning Bolts – you'll probably hit Kessler a time or two as well.
- Lightning Blast
Kessler rears back, charges up and unleashes a huge lightning bolt. You can easily dodge the bolt as he takes several seconds to charge it up; just press 
and move to the side to get out of the way. This is your opportunity to really shave off some of Kessler's health. After four blasts (three blasts once he's down to 75% health), Kessler kneels down and takes a breather. Hit him hard. If you're playing on Easy or Medium, use Megawatt Hammer. These don't work on Hard because Kessler dodges the Hammers.
On Hard mode, you'll have to use Shock Grenades. Try to time the blasts and keep your aiming reticule on Kessler's current location (even as you dodge the blast). Kessler will release four blasts in a row until he gets below 75% health; then, the pattern is three blasts in a row. As soon as you have dodged the final blast, throw three or four Grenades at Kessler's location. Hit him with Lightning Bolts while he's down and he should also take a decent amount of damage from the Grenades.
- Winning
Throughout the battle, you can get quick recharges at the sparking wires sticking out of the ground. When Kessler reaches 75% health, he will grab you and you will engage in the same grabbing mini-game you went through with Sasha. You can't win this mini-game, so don't worry about it.
At 25% health, Kessler summons a trio of the hover bombers. This makes the last quarter of your fight very interesting. Release a Lightning Storm as soon as Kessler summons the bombers and try to hit them before they start flying around. If you miss one or two, you'll need to run around in circles trying to shoot them down or they'll kill you while you're playing with Kessler.
Once Kessler has lost all his health, you will once again play the grabbing mini-game. Win and that's it! If you lose, Kessler gets some of his health back and you'll have to "kill" him again.
Watch the final cutscenes and credits and then go back and collect Blast Shards, Dead Drops and finish up side missions. You can then try again taking the opposite Karma path (and trying to earn the Hard Finish trophy, if you didn't play through the first time on Hard).