The Giant:

Step 1 - Pack-a-Punch ANY Weapon of your Choice

You must get an upgraded weapon from the Pack-A-Punch Machine.

Step 2 - Shoot the Control Panel outside the map

Shoot this weapon on a console in the back of the map. It's located opposite of the animal testing labs entrance.

If you hit it, a number of items will glow green and fly into the air. You must now find these items around the map in any order and shoot them with any weapon.

Step 3 - Find the Teddy Bear Locations Around the Map

One item is located within the right furnace under Teleporter A. This is near the Animal Testing Labs. The furnace will not be lit.

One item is located on the fire escape of a building outside the map. This building is left of the power switch.

One item is located within the left water cooling vat in the second floor generators room above the hangar. You will need to jump to see it properly.

Step 4 - Acquire the Annihilator

After shooting all three items, the Annihilator will be available in the furnace room within the hangar. This weapon is a special weapon and will not take up a normal weapon slot.