Since [Grendel] is able to do everything while being in his 3, this build makes heavy use of this and builds around it. Note that this build also uses a single Umbral Forma and that adding more into him opens up some options.

Aura: With a small shield, Catalysing Shields paired with [Brief Respite] will allow for easy, abundant shieldgating.
Exilus Slot: Since we're planning to always be in Pulverise, there's no reason to use Primed Surefooted and instead use his [Catapult] for greater mobility.
Range Mods: [Stretch] and especially [Overextended] are largely to [Extend] the radius of his 4's splash radius. The loss in strength will eventually be made up with Arcanes.
Umbral Mods: The full sweep. [Grendel Prime] has enough armor that using [Umbral Fiber] makes him a thicc boy. Combined with the Set Effect, [Augur Secrets], and capped Molt Augmented, you'll be able to hit 200% strength for full armor strip on his 3.
Flex mods: [Primed Flow] can be swapped out if you really don't need that large of an energy pool. You could replace it with [Adaptation]. However, on an empty stomach, you're already get 78% DR and with a full belly, [Grendel] will already get 88% DR to his health which doesn't need to be stacked up. Combined with shield gating, you should be able to safely take plenty of beatings.
Arcanes: Molt Augmented is basically needed in order to hit the 200% Power Strength to fully strip armor in his 3. Blessing can be swapped out with your choice, but I prefer the added health to be able to take ever more damage.