[Grendel]'s kit already works well without significant stat investment due to the way it scales with enemy level. This allows us to spend a lot of mods on more survivability to make him a nearly unkillable tank.

Fire Blast is subsumed because it works well with the neutral build stats but this build works with a wide array of other abilities, including his base. Most of these abilities will only offer QoL because he doesn't have the stats for abusing them. We replace Pulverize because it is usually the weakest ability but if you want to be more of a support tank you should replace Regurgitate.

Strength is increased a bit to buff all aspects of Nourish. You get more health, energy, or damage and it's the primary ability you'll be spamming to abuse the shield gating with [Brief Respite].

Duration isn't needed because you'll be spamming Nourish and the drain from Feast is easily counteracted by the massive amount of energy you'll be getting back.

Efficiency is intentionally kept at 100 so that Nourish will give you all of your shield back from [Brief Respite]. If you run a Decaying Dragon Key you can run [Streamline] over [Augur Secrets] but the polarity plan may require additional forma.

Range doesn't need to change because your abilities have plenty of range already.

[Brief Respite] allows [Grendel] to get a significant portion of his shields back, making his shield gates last longer. You can abuse this even further by running a Decaying Dragon Key.

[Primed Sure Footed] is one of the strongest QoL mods in the game and when you're planning on taking a lot of hits you should probably run it.

[Quick Thinking] synergizes with Nourish because you can easily refill your entire health bar with a single cast. The build hinges on taking a lot of punishment so you sometimes need an additional buffer against extremely high damage attacks.

[Hunter Adrenaline] and Nourish's energy buff will keep your energy topped up. [Rage] can also be run but the Nourish synergy gives you such an insane amount of energy that it usually isn't necessary.

[Vitality] and [Adaptation] help our survivability but can be tweaked to other mods based on the content. [Adaptation] should be switched to [Rolling Guard] if you're going against certain problem enemies but remember that this particular build doesn't scale into four digit enemy level endurance runs.

[Natural Talent] helps your abilities to feel more fluid. It synergizes with several aspects of the kit including [Brief Respite] and Nourish spam.

Arcane Grace and Arcane [Guardian] increase our survivability and probably shouldn't be replaced. You can run Energize over Grace if you're having issues with enemies not doing enough damage to you to abuse the Hunter Adrenaline/Nourish/[Quick Thinking] gimmick.

NOTE:
[Grendel] is naturally a tank and you shouldn't need to use this build to tank anything. Especially because of his unique ease with abusing shield gating and [Brief Respite]. But he is the only frame that can buff the energy you gain from [Rage] mods which allows him to create an infinite loop by spamming Nourish no matter the level of the content. This requires a good understanding of how much damage the enemies at specific levels are able to do so you know your limits in scaled content.