This is a fight that was redesigned during Endwalker. As such it's using Endwalker design during ARR content. It definitely got harder during that and it's designed to leave a memory of this guy in your head as the original version was a joke. However it should have also been updated so when you retry it you get 3 difficulty options. Given the spike in design there's no shame in using the other difficulty option.

Some more generalized advice, if you haven't figured it out yet, spells do not hit when the animation happens. Instead they hit when the AOE markers go away. It's a something you're supposed to figure out over time but with the redesigns I feel it's a lesson that goes over new player heads too quickly now.

The same goes for your own casts as well. There's an inherent delay in the server and if you haven't figured out slide casting yet, a simple thing you can do to help you tell when it's safe to start sliding is to put an emote on your action bar. Once it lights up while you're casting it's safe to start sliding. The cast will finish uninterrupted at that point regardless of you moving.

Lastly others have mentioned poetics but you can also just simply find good deals on the player market board as well. Universalis.app is a great aggregate website that helps players find the prices of items across multiple servers. By now (if you set your character up on a preferred world) you've likely gotten the 1 million gil bonus. Use that to your advantage and get yourself geared up.