are you kidding me, raid bosses should be the one's who one-shotting you, not low level bosses, its so frustrating getting half ur HP drained from one hit and the another to finish u off when u don't have a nice stack of rez T.T why do u all think its enjoyable and rewarding to be killed easily, are u all masochists?
This sounds elitist, but I really don't mean it that way - learn to play.
That's the tl;dr.
Everyone has to start somewhere, I get that. I'm not a veteran myself, even though I've played on JP before (didn't get far however). I guess a good way to start would be to pay attention to boss animations. Bosses, as well as some monster, have Super Armor. Meaning that they won't flinch when you hit them. You have to be extra careful, don't get greedy with damage dealing. It's better to take two steps back to prevent a hit, instead of going all out in hopes to kill the enemy.
You have a ddge mechanic for every character. It's different for every character. Learn to utilize it properly. Don't mindlessly dodge, play it safe. You dodging is bound to your stamina, so make sure to have an eye on that. Trial and error. You will get the hang of it, I'm sure about that.
Also, most attacks have visible indicators on the ground. Make sure you avoid those! Red means danger!
If you still feel like it's too hard, play in party and bring a Stella along. Stella can switch her stance with right click and send out powerful healing spells, most prominent ones being Phantom Trail and Orb Song.
That's all I can think of for now. If you still struggle, maybe check the Guides section or watch some Youtube videos.