The weekly raid pits your party against a boss that changes with the calendar week. You face a boss scaled to your party, earn points for the health you remove within the time limit, and gain a time bonus for a kill.
Open "Weekly Raid" in "Dungeon" to see the boss and remaining attempts. Open the party selector, choose your mercenaries, and start a challenge.
Use the raid to compare party performance without injuring your mercenaries. Gold rewards cover improvements to your best score for the week, so review the result and adjust roles before spending another attempt.