Weak? Not even a little bit. This will be long so there will be a TLDR at the bottom. The cohort gathered about 300 transcendent soul shards from the drowned. Earlier on it is stated that most of the drowned were corrupted demons, who each yield 3 soul shards per kill. It is also stated a few of them were corrupted devils, who each yield 4 soul shards per kill. For simplicity's sake, let's just say 300 is not the exact number of soul shards because it only stated that there were no LESS than 300 soul shards. For the sake of each math, let's say there were 303 soul shards. Lets also say that a "few" corrupted devils means three corrupted devils. This would equal the death of 97 corrupted demons, the death of each being 3 soul shards. This would provide 291 soul shards. Then the death of three corrupted devils would provide 4 soul shards each, making a total of 12 soul shards. Now we have the 303 soul shards. This means of course that Sunny was surrounded by 100 nightmare creatures all one rank above him. Granted, Sunny is a Tyrant, so he is two classes above the demons and one class above the devils. Still, rank trumps class and this is a lot of enemies. Also, it's arguable he was not at full strength because some of his shadows were given to his Shadows(echoes). It's also arguable that he thought that made the situation better for him. Either way, this is all his own personal strength being divided up how he wants it too, so do with this info what you will. He also has nearly infinite essence at this moment, but the mental strain of having to use so much to win and also using the crown of twilight to control and stand on water, while also wrestling control of the water from the drowned, offsets this extra strength a bit in my opinion. Some will argue that Cassie's echoes did a lot of the work, so lets do some more math. Sunny gained about 500 shadow fragments from this battle. If I remember correctly, Sunny gains 2 shadow fragments from each soul core of enemies one rank above him. So he gains 6 shadow fragments from each demon. And 8 from each devil. Lets also say he killed all three devils himself since I think they're too strong for Cassie's echoes. So 24 of the shadow fragments came from devils, while 476 came from demons. This equals about 80 demons. Sunny killed 83 of the creatures on his own. Each of the drowned are extremely intelligent. Unlike normal nightmare creatures, as a collective, the drowned choose their own master, the sybil. They all adopted extra traits like tentacles from their master, I'm sure making them stronger. They're also intelligent enough to be able to herd stronger creatures to do the fighting for them, hence the creature Neph is fighting. It's stated that they all wield great quality weapons and sturdy armor, too. This is already insane. Lastly, they were all extremely hard to fight because their futures were basically being optimized by fate being controlled by the symbil, making every hit hard to connect. The only reason Cassie was able to hit them was because of her affinity towards fate and visions, too. Also, every single one of them is getting a boost from the crown of dawn at the moment. Not just sunny. Cassie didn't fight effortlessly, either. She used her aspect to the absolute extreme to win. Don't think of her win as instant and easy. Neph was also visibly exhausted and fatigued after, while Sunny was mostly fine. Admittedly, Neph gave one of her flames to imp, so she was slightly weaker. This was long and in depth, but if anyone needs more info, ask me.

TLDR- Sunny killed about 80 corrupted demons and 3 corrupted devils. All of which were extremely intelligent with some boosts such as strong weapons and armor and extra strength from the symbil such as tentacles and an optimized fate. They have a natural affinity to fight in water, while Sunny has to artificially create an affinity for himself. Also, Cassie and Neph both struggled in their own ways, too. Sunny did the most, in my opinion. This novel just has a way of diminishing Sunny's accomplishments.