Due to their superior speed and agility, Agni and Rudra have a high chance of beating Beowulf in a fight.
The major source of Beowulf's strength is his size. A hulking giant of a demon, his fists pack a lot of punch and he can tank most attacks. His only physical weakness is his eye. His dash attack utilizes his endurance and size to run over his opponents. His light attacks help back opponents away with a radial blast or take then from afar with fast feathers of light. Lastly, by stomping the ground, he causes large torture chambers to fall and throws them at his for, using his environment to his advantage.
The twins, however, are quite quick on their feet and are about as durable as he is, flinching only when their attacks are parried. Their elemental attacks give them a lot of versatility, ranging from powered up slashes to large gusts of wind and blasts of fire. Their major weakness is their tendency to get in the way of each other's attack but this is due to their strategy of surrounding and flanking their opponent.
The major drawback for Beowulf is his size. If the area is too small, then he would have trouble moving around. His light attacks are his only real advantage but with his eye being his major weakness, he would have a hard time using his light powers while preventing his opponents from getting to his eye. If they do get to his eye, then he becomes blind for a short period of time or permanently depending on how deep they cut it.
The twins however would have no issue in this fight. Their agility and speed gives them quite an edge and their strategy of surrounding and flanking, utilising their strength in number, are a perfect counter to Beowulf's size. Beowulf would be forced to use his elemental abilities to keep up but that's being a lot. The twins can deflect his feathers and stay at a safe distance away from his radial blasts and only need to strike when the time is right.
So, my money would be on Agni and Rudra.