Sudden Impact is the third mission in Crysis 2. For this walkthrough, players will continue to move towards Nathan Gould's location, while encountering more human and now alien resistance.  This mission is also significant for introducing weapon and suit upgrades.

1. Make your way down the hallway until you come to the walkway. 
2. You will see an alien spaceship crash into a building ahead. Wait for the mission update from Gould, and head to the left, into the parking lot, towards the blue mission marker.
3. Lift the gate by pressing F.
4. The easiest way to deal with the 2 soldiers ahead is to stealth kill them both. Remember that stealth kills can only be made from behind the  target in stealth, not in front.
5. Stealth kill the soldier past the gate to the right as well.
6. Move up to the higher floors of the parking building by following the spiral. As usual, decide what to do with the soldiers on your way to the quest objective. The recommended option here would be to stealth past some, and kill those that can see you recharging your cloak energy.
7. When you reach the objective, you will be given the option to customize your weapon for the first time. This a standard feature in the Crysis series, which allows you to modify weapons to suit your style of combat. Make any changes you wish by pressing X on the weapon you are holding, and then choosing any available upgrades/changes.
8. Walk onto the landing ahead with cloak on. Here, you will see another large open area, similar to the situation in the open square in Mission 2. Once again, use the visor if you wish, and decide your method of approach. The approach for this walkthrough was to take the right from the rooftops, and make your way around the building, killing a few soldiers and getting to the crash site quickly and with minimal aggression.
9. You will get a mission update. Examine the alien pods which will show up as blue markers on your mini-map.
10. Note: you can examine the pods while still in stealth mode, so it's possible to do this without any fighting at all.
11. When you get close to the ship, you will discover that you can't really get to the blue markers (pods). Wait until the mission update, and then head to the elevator door guarded by 3 soldiers up above.
12. Force open the elevator doors. As usual, you can do this while still in stealth, but it's a good idea to bait the soldiers away from the area first, so they don't notice you opening the doors right in their face. To move them away from the doors, just shoot at them from below and then move up in stealth.
13. Navigate the sewer into the ship. Once again, there's only 1 way you can go so you won't get lost.
14. Go up into the clearing covered by the alien goo, and press B. Focus your screen on the blue marker and press F.
15. Jump out through the window and take the rocket launcher along with the ammo.
16. Head out through the opening above.
17. After the short cutscene, kill the alien jumping around. It will take more hits than soldiers, but it doesn't have that much health. If you have a problem following or spotting it, use nanovision as advised in the tip.
18. Now, you will gain the ability to start customizing your suit. Toggle on the only upgrade you have right now, and close the menu. Much like the weapon modifying, this will allow you to modify your gear to best suit your playstyle, or a specific situation you're in. To get more upgrades, you will have to obtain more nano goo from the alien corpses, and then use it to purchase an upgrade you want.
19. Head towards the newly-marked mission objective.
20. Note: you can sprint here without stealth, as all the soldiers are now dead.
21. Drop down into the underpass and continue on.
22. Seeing how the soldiers ahead are a little busy, you can easily take them out with any method OR choose to help them with the aliens first. 
23. Jump up through the small opening, just like the alien did ahead of you.
24. Note: make sure to walk over any alien bodies to automatically loot the nano catalyst for more upgrades.
25. When you're near the objective, take the second doorway on your right instead of the first one.