The walkthrough writers give a good description on completing the level:
Head all the way left and forward, defeat the Stormtroopers, and as Finn use the lightsaber on the dotted red line in the door to break it. Head out and use the saber on the horizontal dotted red line across the grille hiding a small grate. Go in there as Maz, smash the boxes and rebuild the checkered path to push the block off the edge. Jump down and multi-build left to crush the Flametrooper. Then multi-build right to blow up the other one.
Blaster Battle time, and another opportunity to get any of the related achievements mentioned in the Prologue. To move on, focus specifically on the heavy gunner with the health bar. Once he's down and the cutscene is done, you need to kill five Stormtroopers while avoiding being hit by a TIE fighter missile, which appears as a red circle under you for a few seconds before it hits, giving you time to move. You should then use the grapple point on the right to get some height, from which you can target the sniper and the turret (shoot the turret, not the trooper, for the hits to register). Mop up remaining Stormtroopers to move forward. Access the scan point on the right as Han and reveal a shiny metal area at the base of the statue on the right, which you can blow up with Chewie's Thermal to crush the turret. In the final area, you can hop on the turret in the centre to finish off the two Flametroopers that appear. Blaster Battle done. Head up the stairs and shoot a few Stormtroopers to eventually reveal a boss Stormtrooper. Smack him around when he's dizzy and he'll smash up some stuff. You need to build each of the three multi-build options in any order to deal some damage to him, smacking him around a bit in between each build. Once you've beaten him up three times, mash the button prompted during the clash between the Stormtrooper and Finn and then watch some cut-scenes.
Now it's time for a dogfight as Poe. This is straightforward enough - take out 15 TIE fighters first.
Red Brick - After you have destroyed the TIEs, the eagle-eyed among you may have noticed that a Red Brick appeared right at the end of the cutscene, on top of the now-broken pillar. You might as well pick it up now, just fly into it (it's in the middle of the smoky ruins on land in the centre, quite easy to spot).
Destroy 10 of the large transport ships, indicated by red arrows. Then help out the command shuttle by taking down the groups of Elite TIEs chasing it, again indicated in red.
Here is a video walkthrough:
Credit to HappyThumbsGaming.