This build is meant to turn [Grendel Prime] into a veritable behemoth, tanking through damage with high damage resistance and pulverizing foes, heedless of their armor.

What Archon Shards should I use? My recommendation would be at least two Health Shards, with the remaining shards being up to preference. Two Duration Shards can help [Extend] the uptime of Nourish, Weapon Buff Shards can be useful for [Grendel] since he relies more on his weapons than his abilities to kill swathes of enemies, and Health Regeneration isn't a bad idea either.

The only Shards I would advise against using are Power Strength, Armor, and Shields. Increasing Shields may be useful for non-Catalyzed Shield Gating, but this is an armor-tank build. Stacking further armor will provide violently diminishing returns, as the combined Armor bonuses put you at 93% DR, meaning any further margins to gain would require ridiculous armor buffs, making Health a preferable defensive Shard stat. And Power Strength beyond 200% isn't needed, as the primary purpose of it is to guarantee 100% Armor Strip with both Pulverize and Regurgitate.