Crysis Warhead/Adapt or Perish

Colonel Lee has something that could be devastating to mankind. You must stop him.

Pursue Colonel Lee
Lee has escaped with the container. Follow Emerson's instructions to capture him.

Wait until the big freaky thing is gone before you jump out of the sub, grabbing some nano-disrupters along the way. There are some weapons right outside the sub, but there are also some KPA in nano-suits. Conveniently, we have nano-disrupters, so use them after grabbing the weapons.

Take hovercraft
Grab the hovercraft for some needed speed on the ice. You'll have to take down the soldiers. It's slippery as hell and the camera sucks, so take a bit of time to get accustomed to it.

Follow your objective, but watch out for aliens and mines (sea mines, technically). The mines have a good chance of killing you outright, and the aliens will give chase and sap your health. Simply follow what Emerson is saying.

At one point you'll see a group of KPA fighting a massive spider alien thing. This is where you get out. Find the stairs that lead up the ridge, and follow them.

NOTE: If you make several stops along the way, it is possible to acquire better weapons than you would normally have, like FY71s, Percision Rifles, and LAWs.

Rendezvous with Eagle Team
Rendezvous with Eagle Team. You will need to leave the hovercraft behind.

You'll find your first encounter with what I call the hoppers, small aliens that are faster and smaller than the others. Thankfully they're also weaker, but they still explode after they've been defeated. Sometimes you'll see one or two covered in an impenetrable shield: this means that there's another nearby generating that shield, so you'll want to take that one down first. Don’t forget that these aliens can be grabbed and beaten like the KPA infantry. Make your way around them any way you can. Keep going and you'll run into Eagle Team, another group of nano-suit-equipped commandos.

Reach the tunnel
There's a soft spot in the ice leading through to a mountain tunnel. Reach it to move to the next valley.

Eagle Team is going to be a lot of help here, taking care of a lot of the enemies you encounter. Just keep pushing forward and they'll follow. Head along the dirt road, but try not to stay in the open. Since you're in the front, more often than not you'll be the target, and those ice beams hurt. There’s also going to be one or more flyers, so watch the skies.

When the path turns to east, on the western side there's a cache of ammo just as some KPA vehicles turn the corner. Grab the grenade launcher in the cache and unload on KPA before moving forward. You'll enter a large area filled with destroyed vehicles, weapons, and ammo. Grab what you can, and get ready for a big battle.

Destroy the Exosuit
The exosuit stands between you and the tunnel entrance. Destroy it.

Hold your fire until you see the giant alien open its mouth, then fire what you can into its jaw. Make sure you keep moving, and take care of any other aliens that arrive and attack your team. Use whatever you can, just keep attacking, and make sure you don't get caught in its ice beam.

Enter the tunnel
Enter tunnel once Eagle Team have blown door. Head through the tunnel to proceed.