Hot take, but - while its definitely not the worst main boss of the game - I do think Elden Beast is one of the weakest design-wise (gameplay wise) bosses in Elden Ring, at least prior to the patch that allows you to use Torrent in the fight. Mostly because it keeps teleporting away and you have to constantly give chase and this is the very final boss of a very long game. It should be a very climatic fight, but the manner its done makes it very underwhelming imo.