The Giant Spider has a lot of health and deals a lot of damage with its Leg Stab. The room you fight it in is also quite small, so you do not have a lot of space to dodge attacks, meaning taking the spider down quickly is ideal. The Dingo LMG is very good at taking down the spider once its weak point is exposed.

You'll know when to shoot its weak point when its mouth is glowing and foaming. Once you see that, unload as much damage as you can into the mouth of the spider until it is stunned. Using the Dingo, it takes about 3 mags to fully take down the spider on solo.

Throughout the boss fight, regular spiders will join the battle, which you can mow down quickly to get a Max Ammo Power-Up after killing a few of them. As long as you avoid the Giant Spider's attacks (especially the Web Projectile, as it slows you down), you will take down the spider pretty quickly.