The Final Encounter

Stand behind the white Conversion Gel pipe as soon as Wheatley starts talking. He will throw mines, shattering the pipe and coating the floor in Conversion Gel.

Place a portal on the white wall up and to the right of Wheatley, and another portal not too far in front of him. Stand just behind the portal on the floor. When Wheatley throws mines, step back, and they will return to hit him.

Step through the portal to the upper ledge behind Wheatley. Go to GLaDOS and pick up the corrupted core. Use the blue Repulsion Gel to the left of Wheatley to bounce up and place the corrupted core on him.

Leave the floor portal as is. Place another portal directly under Wheatley. Stand near the closest portal, and Wheatley will throw mines into it, hitting him from below.

Place a portal on the wall behind Wheatley (the same one as before). Walk to the end of the broken walkway and drop down. Bounce up to catch the second corrupted core. Use the blue gel in the middle of the room to reach Wheatley and attach the core to the right side of him.

Do not move the portals. Stand behind the ground portal and move back slightly as mines are thrown. Wheatley will expose his back, allowing you to damage him.

Turn left and find the wall covered in white Conversion Gel. Set a portal here and another on the ground to its left. Once the orange Propulsion Gel coats the ground, run through the portals to grab the final corrupted core at the peak of your jump.

Use the central blue gel puddle to bounce up to Wheatley and attach the final corrupted core.

After the rain, wait for it to clear. Place a portal on the remaining patch of white gel. Make your way to the stalemate button.

Your view will lock onto the ceiling, showing the moon. Fire your portal gun at the moon to trigger the end sequence.