(You can use [Power Drift] instead [Catapult], it depends on your playstyle though)
How to make [Grendel] OP?
Step 0: Follow this build (it may be less or more accurate xD)
Step 1: Use Gloom
Step 2: Use Feast to devour enemies
Step 3: Throw enemies up. The more enemies you eat, the more damage you'll deal to them (Protip: Feast's damage scales with enemies' levels). Once you'll vomit when you have Gloom active, you'll receive 100% of HP back (at most cases), which allows you to use Feast again.
Step 4: Repeat until you pass mission and recast Gloom if energy runs out.*
Step 5: GROFIT
[Rolling Guard] is useful when your HP is extremely low and you don't have enough energy to keep Gloom active. Also this removes any bad Proc like Slash, Toxin or Burn
I also recommend to have Decaying Dragon Key equipped for Shield Gating.
*You can't eat bosses, enemies from Index/Arena (Rathuum) missions, Eximus units in SO.
You can use Blood Altar as alternative Helminth ability, but the tactic is the similar to [Spectrosiphon] one.