The Ash skin is found deep in the Dungeon of Eternal Life. There are golden doors around that block your progress. Sneaky placed lanterns are on either sides of the doors. Light them with Ash, and eventually you will find the new, blue skin.

Venture to the Ancient Tower. If you do not know where that is, find a long stairway in the Enchanted Kingdom that leads up to the throne room. Go left to find a passage leading to the Ancient Tower. There is a squid person there who needs you to round up her children. Equip Major Ruin, Chilla, and Ash to do the tasks. You will find a ramp that Major Ruin can go up. Then, use Chilla. You must repeat this process every level along the way. On every level is a squid child. They will be frozen in a block of ice. Use Ash to melt the ice and the kids will go to their mother. Continue up the ice until you reach the top. The little bug things will make it difficult all the way up, but it is possible if you keep trying. Once you get to the top, you will find the last squid child and be rewarded with three elemental fruits, as well as the new Chilla skin. Get the skin, then work your way to the bottom. When you get there, collect your fruit.

The Flex skin can be found at the Ice place. Venture around until you find the shop that you do not have to walk into. It costs a bit, so you might want to have a full wallet.

The skin for Major Ruin is in the water place shop.

The Pummel Weed skin is in the Enchanted Kingdom shop.

The Rubble skin can be found in the forest place. Again, find the shop and buy it.