Shalltear is stronger then cocytus in a one v one context but cocytus is actually physically stronger then her, and does more dps overall, he lacks sustain/magical abilities (He weilds four divine class relics, on top of his tail, giving him five weapons/sources of damage when in melee). People really misunderstand the strength of shalltear, she's the strongest/most optimized of the guardians but the gaps aren't massive and all the guardians are better at different things. For instance mare has the highest aoe damage, and would actually beat shalltear if he kept to range, aura and her beast are similarly powerful/dangerous. They're all designed to defend their floors from hordes of high level players, none of them are weak compared to the others, outside victim, whom exist solely to die.
Feat wise cocytus has only two of his own, both from the great evil tree audio drama (Which was confirmed cannon when the dryad from that story appeared in volume eight) Smashing two 300-meter-long tentacles with a single strike, and setting up to one shot the 100-meter tree kaiju using an ability called wisdom king strike from a good distance. Albedo tanked the attack to stop cocytus from killing the monster instantly, which only injured her hand.
I guess he also stated he could have frozen the entire lizardman village and had to restrict the damage and area of his aura not to insta kill the lizardmen cheifs?
A fact of overlord is outside ainz and shalltear we've seen very little of what the gardians are capable of, we just learned mare can turn their body into an elemental state and self revive using a flame type spell. He's also apparently really strong in melee likely aping the dnd druid is a better fighter memes.
Like sebas just randomly has a mind control skill in his pure dragonoid monk build...For some reason.
Really using anyone outside ainz and shalltear from the level 100's seems rather pointless to me, we have so little on them there's not much that can even be discussed beyond stats/scaling. You could also use pandoras actor I suppose but he was massively holding back in his pdl fight, and is basically just a weaker ainz since we've never seen him use any other form in combat. Theoretically he should have the most varied abilities and stats with his forty one forms but we've never seen them.