Aion 2 has several dungeon names that sound like variations of the same activity. They are better treated as different lessons. Solo content gives you room to learn your class. Expedition teaches group consistency. Transcendence asks more from the party. Sanctuary is not where you should first discover that you missed a basic defensive skill.
Use solo content as a diagnostic
Nightmare and Ascension Trial offer a place to test timing, positioning and survivability without asking a party to carry the experiment. If a boss keeps catching you during a long attack animation, change your rhythm before changing your gear. Chapter 1 added higher solo difficulty, so the menu offers more than a single pass/fail checkpoint.
Move up when the group is ready
Chapter 1 expanded Expedition and Transcendence parties to five and Sanctuary to ten. Check the entry requirements of the specific dungeon; for example, Citadel of the Fallen Daeva and Abyssal Horn Den were introduced with level 50 and item level 3,500 minimums. Those figures are examples, not a universal ladder. Before a first clear, agree on roles, recovery resources and whether the run is for practice or a reward claim.
A clean practice run is a valid step on the ladder even when you cannot open the reward you want. If your group repeatedly reaches the same boss phase, discuss that phase, not a new class composition. If the party cannot survive the first pull, return to an easier dungeon to establish timing. This route creates progress you can feel without pretending every failed run needs a purchase.