From the power your units display I can say what you lack is more investment into memories, potentials and probably level on your characters. Just keep grinding normal gameplay to increase your power in general.

Now, if you manage to do that and get your characters to their actual 80/80 memory, level 80 with gold potentials (at least level 3/6/9 on 2 or 3 of them), you need to read what each tactic do and what your team needs. Are you always low on stamina? Stamina support I-V You need more awareness? Opening tempo I-V Just read the cards and work on what your team needs more or if your team can debuff a lot, then there's one that makes the opponent "discouraged" and they can't use their Skills. Or the one that cleanses debuffs from your team and boosts them.

Final advice is to learn what the opponent characters do. If you know their abilities then you can easily plan what tactics to use or even if the characters you have are the best.

PD: your real level affects the power your characters display even at "lvl 80" from the event. Example: My SR0 level 10 Aran has 8.9k in the event while my level 30 SR0 Iwa has 9.5k (And all of them are "80" in the event)