To enter the Guild bosses make sure you have enough Guild member or your have strong character to conquer boss at the this Dungeon.
How to join Guild bosses:
After entering to the Guild bosses Dungeon, tap icon to display and select the Guild bosses you want to become a opponent. Make sure suitable to select opponent with your level, if you or other of member Guild die before cleared the Guild bosses, can participation again with use item Resurrection Stone or spend with 50 Crystal.
Select one the Guild bosses you want and tap Summon for starting the battle. More increasing boss level you choose, then the harder you can beat it.
Get reward : Gold and Exp!