Well, if Fuyu allows it you could use Ophanimon. It can technically be tracked to one of the possible digivolution lines of Impmon (specifically Impmon->Gatomon->Angewomon->Ophanimon). As far as Ancient Mermaimon, she could easily be considered OP. Her data was used to create both the Human Spirit of Water and the Beast Spirit of Water, which should mean that she's a pretty strong digimon. Additionally this is a quote from DMA "One of the Legendary Ten Warriors who saved the Ancient Digital World: this Digimon bears the attribute of 'Water'. One of the first Mega-leveled beings to have existed since ancient times, it is the protector of the Net Ocean, one of many Digimon birthplaces. It rules over every inch of the vast Net Ocean, and it can manipulate water as if they were hands and feet, creating currents and tsunamis. It's been rumored, but not confirmed, that AncientMermaimon's anger can sink islands and continents with just one touch. Many Aquatic type Digimon consider AncientMermaimon to be their goddess, and have thus inherited her abilities. Its special attacks are "Crystal Billow", which fires out super-hard crystals like a machine gun, and "Great Maelstrom", which generates a huge maelstrom that engulfs everything in its path in water."