Child of Light

During chapter 6 you will find yourself in the town of the bolmus populi. You must make your way to the far left of the town and enter what looks like a palace. There you will talk to a rat who will come across rather rude. He will then tell you to leave, which you must do so. Outside you will run into Robert the Rat (he is an awesome archer) and he will ask you to help him get inside the vault that lies deep within a giant which you may have seen when you were flying up to the town. You will be taken there upon him joining your party, and will have to travel with him into the deepest part of this giant. Now deep inside here there is a big boss spider which you will need to remove to continue with the story. To fight the boss you will have to clear all 3 spider lairs (they are pretty tough so be prepared).
I recomend using sister power to fight this;

Using Norah's unstoppable first to prevent interruption since the spiders can cast really fast.
Then use elevate damage with aurora to give yourself 25% more power for 4 turns.
Then use Norah's quicken spell to give both Norah and aurora speed ups to cast faster.
Then use starlight for good damage, slash (all) if You have it and Norah's petrify to stun the spiders.
Rinse and repeat.
Once you beat the 3 spiders the boss can be fought so go back to the large Purple Heart and challenge him!
The boss hates the Starlight so work on taking the other 2 out using the same tactic as before but using star light and elevate more on the boss.

After this go to the vault, you will need to play a lever game and get the right combonation then after that use your navi look alike friend to light up the rings and the door will open. Once inside lay 4 chest which you may open and it will complete the mission Roberts Plight. Roberts default skills include;
Shoot arrow: shoot an arrow at a targeted dark creature
Hinder: shoot a magical arrow that reduces a targeted dark creatures speed by 50%
Antidote: cure status alliments of a targeted ally

During chapter 6 you will run into a town called Bolmus Populi. Head all the way to the right of the town and enter the golden gate. You will talk to a rather rude rat, and then be asked to leave. When you leave you will meet Robert and he will tell you he can get you to the towers in the sky. The achievement will not unlock as soon as you meet him.

After this you will teleport to the Mountain Giant (I forgot his name) right outside of town. He will open up his mouth for you to go inside. You will then have to make your way down to the bottom. YOU DO NOT NEED TO FIGHT ANY BATTLES. You will need to fight the spiders to continue the story, but not for the achievement.

Head as far right as you can in the tunnel and then drop. You will have to avoid one spider. There will be a bridge in the background and a very short cut scene. After this, drop straight down into the bottom. At the lava pool go left and you will be at the door.

The lever combination is Right, Left, Right, Right. Then with Igniculus, hold while following the spinning lights to light them up. The door will open followed by a short cut scene. As soon as it finishes the achievement should pop.

Good luck, hope this helps!