Pretty easy boss but you do need to be aware of the gimmick. On the long path leading up to the fog door you’re able to go up each side and walk along the balconies. On the end of the right hand side balcony will be a non hostile NPC. Kill him or he’ll continue to resurrect the boss.
The fight itself is very simple. After you attack the boss it’ll disappear and then reappear with multiple clones of itself. All the enemies will fire magic projectiles at you. If you look carefully you’ll see that one fires a slightly different, wider projectile at you. This is the real Idol. This is the one you’ll need to attack to deplete it’s health as the copies don’t cause damage to the boss health bar. Avoid the projectiles, be careful of magic traps on the floor and make sure you identify the right Idol and you’ll win this fight easily