Instructions
Travel to the Trash Coast, southeast of the Rust Commons. The region is overrun with spiderants, so you're likely to become very familiar with them as you work your way to the tunnel.
Make your way to the southernmost part of the Trash Coast. The Giant and the sewage tunnel are located due south of the region's central spire.
Kill off the weaker spiderants around the tunnel before the Giant engages you. This will allow you to focus on killing her instead of watching your back.
Close in as soon as the Giant makes her appearance. She has strong projectile attacks that can rip players apart at medium range. For optimal survivability, you'll need to get up close and personal, dodging her massive claws while dealing damage with high area-of-effect weapons such as shotguns.
Avoid her front as the battle progresses. Her front legs are armored, which shield her from damage. Try dodging an attack and then darting in behind her. Any attacks against the Giant's abdomen from behind will result in critical hits and cause massive damage.
Aim for the Giant's abdomen when she raises it above her head. As she gets desperate, the Giant will attempt high-damage attacks by shooting spines from her exposed abdomen. Get a few shots off on her abdomen before diving for cover, and the damage against her will quickly add up.
Switch to an explosive weapon once the Giant is weak and shoot for the ground beneath her body. The splash damage from the ordinance will blast upwards and can score the last critical hits needed to finish her off. Continue this until the Giant has been killed.