I used a +9 Rank 8 Fire Element Weapon against Hell's Warden with PRC bullets.
Before going into the fight, make sure your Tenko is exploring the same area, in the event that you messed up and get knocked out, your Tenko can revive you. Your Tenko will also Purify parts.
Throwing grenades (Triangle + Square) generates a lot of Weapon Gauge. Stand off to the side of Hell's Warden and keep tossing it until you have a Destroyer available.
It's rather important that you get at minimum, 7 destroyers before you focus on gunning Hell's Warden down.
With your first 2 Destroyers, take out the horns and purify them. This will reduce his lightning attack from 3 charges down to 1 (-1 per horn).
Afterward, take out his 4 legs and his tail.
The reason you want to take out all of his limbs at this early stage is because of the PRC bullets. Shooting at the right angle, purposely or not, will strike multiple parts.
For example: Shooting Hell's Warden left leg's weak spot from a specific angle can hit his back right leg's weak spot as well. You don't have to go specifically for these multi-hits, just occasionally hitting them is enough.
For the most part, you don't need to use any charge shots, just PRC bullets against his weaknesses and occasionally hitting multiple spots. The best spot I found to shoot is his left foot. Depending on his movements, how he turns, it often leads to multiple hits. You can hear it based on the sound effects. If you hear only 1-2 hits, the PRC bullet basically passed through Hell's Warden. If you hear 3+ hits, you've essentially maximized your hit. You can tell you hit another part of Hells Warden when you here hit, hit, hit, slight pause, hit, hit
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When Hell's Warden is in his second stage, repeat the process of building up your Weapon Gauge. You can get up to 5 here if you want, but at minimum, you need to do get at least one.
You're aiming for the middle most spout on Hell's Warden's back.
Destroying this will stop his targeted AoE. Anytime you see him shoot off goop into the sky, it will fall on you a few moments later. Destroying it will stop this completely.
Destroying his other spouts will stop them from spitting out attacks. If you're prone to getting hit, probably best to get rid of them.
Then repeat what you were doing before his second stage until he dies.
Very straight forward.