Here is a description from the walkthrough writers of how to complete the level as well as earn some other achievements along the way:
Get ready - there's some Minikits to be grabbed immediately in the flying section. You'll get Minikit 1 for destroying 5 turrets on the ground, and Minikit 2 for destroying three of the larger command ships in the air. There's more than you need of both but even so you'll need to keep an eye out for them rather than just firing blindly.
Other than that, this is just an on-rails vehicle section you need to sit through.
When you reach the castle, make sure to hit the green, red and blue flags on the walls to get Minikit 3. Then you need to destroy the castle by first hitting all of the vertical poles that are acting as wall brackets, and then hitting the conical objects in the flames once the walls have been cracked. Here's a screenshot example of the object to hit, just to the right of centre:
Conical thing
You'll get the following achievement automatically for finishing the section:
Once on foot, use the Dark Force on the red object to the left to break it and build up Minikit 4. Use FN-1044 to pull the orange handles to the right of the cracked LEGO wall with any character to unlock Minikit 5 to the right. Smash the cracked wall and use the Dark Force on the wall behind it to create steps. In the next area, destroy the waves of enemies that come at you. After the tiny cutscene use your lightsaber on the wheel in the back of the area for Minikit 6. Head to the left side and jump up the horizontal poles on the wall. Jump back off the top one onto the platform to claim Minikit 7. Use the Dark Force on the red-and-black door to the right to progress (note that you cannot return to the previous area once you have done this). In the next area, smash the Cracked LEGO wall and use the Force on the results to free the Troopers to the right, and then pull on the grapple hook at the back of the area. Multi-build either way to free the other Troopers to the left. You now have three Troopers - use the Stormtrooper Officer to command them to pull on the four grapples on the statue blocking the way forward. Before progressing, you should use the Dark Force on the statue of Maz to pop the head off and reveal Minikit 10.
Command the troops again as you progress to the right, get them to push the blue object back and then command them all to shoot the gold section. Switch to Kylo and build up the blue debris into a lift. Put FN-1044 on the lift if he doesn't get on automatically, switch back to Kylo and use the Force on the lift. When it's at full height, switch to FN-1044 and quickly jump before the lift descends. Smash the cracked LEGO wall to the left to create a bridge for Kylo. Head up top as Kylo and use your lightsaber on the tree blocking the way.
In the next section, make sure to hit all of the quick-time button prompts for another achievement. I'd suggest mashing the button as it appears as I'm not sure exactly when you're supposed to push it, and there's no penalty for mashing it.
After the last cutscene, you'll complete this achievement.
Here is a video walkthrough:
Credit to HappyThumbsGaming.