It´s not like "You need diamond characters to beat this legendary enemy muajajaja buy packs or you can´t do anything, hohohohhohoho!!"
It´s all about practice.
There is timing in when to attack and when to retreat. You need to learn when to block, like don´t punch all the combo if it didn´t conect, or you will be punished, or when to attack the oponent when thay fail to conect a attact or a BB...
Here in this humble video you can see a 3k Filia Bad Hair Day dealing with a 38k team, and her Special Ability plays a fundamental rol defeating the other team: She (as other Filias) has access to perma bleeds that takes 1% por second each, so 5 bleeds are 5% per second. But make no mistake, because other chars are inmune to bleeds like Blood Bath Eliza, or Valentines with ICU. Many other characters have SA that helps you to defeat the opponent in one way or another like stuning a lot, gaining invencibility or inmunity, doing armor breaking, gaining enrages that makes them way stronger, etc.
By the end of the video, we can see that she has almost no HP, and recives an attack that would have killed her, but instead, recived a Buff because in the team there´s a Valentine who has a support rol and each time someone losses 10% of their HP, she protects them with her marquee Trauma Center. It´s maybe a lot of info, but as you can see having you back covered means a lot in this game.
You can do this!! Welcome to the game!!