First and foremost you need to save/spare Ligamental, Jordan, Stan, General, Galene, Fae and Dragon when given the option at the end of each chapter.

Here's the correct order in which to use them in last chapter:

1. <focus on water> - use General to help you cross the bridge
2. <focus on art> - use Stan to find a solution to the maze on the wall
3. <focus on ice> - use Fae to open the ice door
4. <focus on nature> - use Galene to burn the bushes
5. <focus on darkness> - use Dragon to get rid of the ghosts
6. <focus on fire> - use Jordan to drain the lava
7. Ligamental will come to your help automatically in the next part.

Be aware that you can use the summon only once and it's possible to solve certain puzzles with different summons. For example you can use Fae to cross the bridge but you'll have to use Jordan to open the ice doors which will result in having to use Eli to help you cross the lava, leaving him behind and failing to get the achievement.