This game is a mess. Locked to 30 FPS 1080p, cutscene with desync audio, unbalanced difficulty and so on.
This guide will help you rage a little less than normal.

You can choose every character, but the best I found is Akainu (2 years before or after doesn't matter), or Rogia Users in general.

This is pretty much your choice, but to me the best are these three
Lola - Fury Bar charges over time automatically



Sentomaru - You start with more HP



Kureha - You can cure a good amount of HP

First you need to start with Ability normal combo (X,X,X,X,X while holding RB)
When the opponent is on the ground activate Lola, recharge the Ability bar and do the combo again until you are sure you can 1 hit them with your ultimate.
Activate the Fury Bar with R3 and the use the Ultimate Move with R3.

Enjoy your victory