[Executor]

Grendelkin is just not as great as you think.
He has certain advantages as not having an undead tag to make his vulnerable to certain items like Tartarians, but he still has no basic elemental protection, and has fatigue. Simply put, you can't equip him and protect him from everything, cold/fire/poison/MR/fatigue. Well unless you use some heavy artifacts but than you can't generalize how unstoppable they are.

I though air and fire elementals dealt elemental, as in slight fatigue damage, guess I was wrong. But still, living water would crush a Grendelkin no matter his HP and protection. Two orb lightnings would probably jump his fatigue over 100 and make him vulnerable to just about anything.

Now, if the Grendelkin moves forward and attacks from the start that you can easily stop him with fatigue damage, and cleverly used battle orders. Combine, air damage, fire damage, some paralyzing poison (eg. ghouls), water elementals, piercer to blind him (works regardless of armor), fatigue damage from stellar cascades, etc.
And if he simply chooses to relax 5x while you scripts fail, than send 2 banes with axes of hate, quickness, stone bird and burning pearl for example.

And as far as large battles go, yes he can have all the possible protections from buffs, every elemental protection to 100, high MR, regeneration, reinvigoration... but so can your troops and SC's. You can beat him by routing, by bringing boxing teams, gloves of the gladiator with quickness and a stone bird, you can boost strength with giant strength, or items. You can cast those buffs on troops and cast weapons of sharpness, and quickness. I bet that would butcher a Grendelkin.
Depending on the battle size the Grandelkin might just sit there and wait behind friendly troops and so on.