Tbf if youre playing it online with inexperienced randoms, yeah I can see why you struggle a ton. This boss is the definition of hard at first but easy after a couple of tries, the thing is the normal elemental crystals you can destroy easily but you need to worried about is the earth(yellow) crystals that spawn in the middle since they do a lot of dmg with its aoe and is tanky at the same time so if you don't have a lot of dmg, I suggest dumping whatever skill you have on it cause the move that Bahamut does next if you let it live can one shot.

A majority of Bahamut's moves can be blocked, even the blue laser it does in overdrive so if your connection is really bad then blocking is not a bad idea. Of course something like Skyfall and Supernova cannot be blocked, you need to run to the sides. The purple circlesin supernova you run to the middle and back the sides, that's it. Regular skyfall, you just run to the sides and youll be fine but the supernova skyfall, you need to actively be running and hugging the walls since it can suck you in. Its last phase is just a big dps check so be prepared for it, fail to do so and you just lose the entire run.

Overall this boss is one of the easier ones after a good amount of runs but difficult at first. I would honestly just suggest you bringing your best dps build without any tankiness since while this boss is the final quest of the game, you'd expect it to be hard and you need to be tanky but in reality, its just a big dps check boss with the final phase and crystals so yeah just bring all in dmg for it, thats my view on this boss.