FGO, Mordred Swimsuit Literally unlimited NP work. C...

This is apparently called The Mo-san System

In case you don't want to watch a sixteen minute long video, this uses a combination of Waver to kick things off and then Tamamo+Support Tamamo to allow Surfer Mordred to use her NP every turn.

From the background and the enemies, it appears that this is a battle from the third Halloween event that has 100 backup enemies.

According to the comments of the person who posted this video, it doesn't matter if her NP isn't top-tier because it is seriously not intended for this kind of sustained damage.

damn now I really wanna see

... All I can think of is BlahBlahBlahTIDALWAVE