Approach one spawner
The challenge activates when a player approaches; more participants increase the combat challenge. Before entering, clear a route back and place a supply chest outside the active room. Do not run through several halls and activate neighboring challenges at once. Reference: Minecraft: trial spawner.
A shield helps against arrows, and a level floor helps you deal with Breeze knockback. Cover dangerous gaps and keep healing items near your weapon. Ordinary torches do not disable a trial spawner the way they can suppress a conventional dark mob spawner.
Two keys - two vaults
A regular trial key opens a regular vault; an ominous key opens an ominous vault. Each participant opens the vault once for themselves: another player does not use up your personal attempt. More keys do not let one player repeatedly open the same vault.
The spawner’s 30-minute cooldown concerns another fight, not a reset of your vault loot. You cannot choose the item flashing inside by clicking at exactly the right moment. Mark the vaults you have personally opened.
For a manufactured mace, start with a short workout: Minecraft mace combat: landing a smash attack and surviving a miss.
Start an ominous trial separately
Drinking an ominous bottle gives Bad Omen; near the spawner, it becomes Trial Omen. Ominous fights are harder and introduce extra threats. Complete a regular room first, secure your loot and replenish your equipment.
The heavy core used to craft a mace comes from an ominous vault, so a regular key will not achieve that goal. Do not break the spawner to collect it: it does not drop as an item in normal Survival.
Shield management and its limitations are described separately: Using a shield in Minecraft Java and Bedrock.