How to join Raid in the Guild Challenge?

For join to the Raid in the Guild Challenge, you must follow provision at below:

1. Your character should be at least level 14 to play Raid.
2. You need gold keys to play Raid.
3. Monsters in Raid are stronger than the ones in other dungeon.
4. You can get Soulstones if you defeat Raid monster.
5. You can joind a Raid room that isn’t full.
6. The time limit for Raid is 15 minutes.