So Meliodas has 3000+ years of fighting experience and physical darkness and danmaku with trillion dark to help him out here
Grimgor scales above; character who were born with the instinctual knowledge at birth of a mastery of combat and military tactics and then spend several thousand years locked in unending combat and honing their prowess, these same characters are outskilled by Swordsmasters of Hoeth who individually spend thousands of years mastering swordfighting to the point that they can solo and kill so many people that they choke a river with bodies, all while simultaneously deflecting hails of arrows and crossbow bolts. Swordmasters are utter fodder to Malekith who can kill them by the dozens with little to know effort, and has had over 10,000 years of doing nothing but increasing his skills and knowledge in combat, he is outskilled by Archaon, who mastered every weapon he has ever come across and has fought virtually every type of enemy in Warhammer, who while completely blinded physically and magically, stripped of armour and weapons, was able to murder hundreds of giant monsterous chaos spawn, this was VERY early on in his career too. Grimgor put him on a backfoot when he also had witchsight (analytical prediction that shows thoughts, emotions etc of those around you) AND stupidly powerful precog. Skill is not the debate you want with this character.
Danmaku also doesn't help here, as danmaku is dealt with through skill and dodging, as has stated by the above skill set where he already scales massively above Swordsmaster who are able to deflect entire volleys of projectiles WHILE fighting off countless other foes in melee. With Grimgor even scaling to Katarin, who can danmaku by literally turning the rain itself into razor sharp icicles. Anything that manages to get past that, also has to get through his armour, which has a chance of just raw negating the damage, or heavily reducing the amount of damage he actually takes from the hit, making it all the more inconsequential via his immortality type 2 and regen.
On top of all of this, this is not including his amps, no matter what Meliodas is at the AP and Dura disadvantage and this only gets worse the longer the fight goes on, as Grimgor has SEVERAL passive amps, including his Rage power (he grows stronger and faster the angrier he gets), his accelerated development/RPL will passively make him stronger, faster and larger over time. Finally most importantly, his Waaagh! amp, which will massively spike his power up every couple minutes minimum. Very quickly Meliodas won't even be able to harm him, while any hit he takes from Grimgor at best cleaves him, and at worst eventually obliterates him.