I cannot advance anymore

[Ydex]

Work on a core team that has a complete strategy, based on their skills.

My first good core team’s defense strategy was having two Sentinels with high HP, both of whom can Revive. If at least one of them survived, I could bring my team back. My core offense strategy was the ability to remove Immunity, and apply Permadeath. That’s it. If I could keep Reviving and picking away at your HP, eventually I would kill you when Permadeath is applied.

You might focus on other skills, based on which characters you have. Maybe Healing instead of Reviving, etc. But make sure your team has a COMPLETE strategy. Each team member has to contribute something to the TEAM strategy-- just being a "good character" doesn't matter.