Siege engine
From Destiny 1 Wiki
The Siege Engine is a boss on the easier side seeing as the boss itself isn't a heavy attacker. To start off you need to go to the other side of the map to start it and once started a lot of enemies will spawn in and thats really the problem. To complete this part you must shoot both the turrets off the side of the engines head revealing a red orange core. proceed to destroy that and once destroyed you may now jump onto the boss which knock the wall down starting phase 2 but DO NOT forget about the small enemies or you will get overrun.
Phase 2 starts right away and you will see the boss come to a complete stop and he wont move. On the other side near the next wall will spawn a huge amount of enemies and a few machine parts. kill the enemies and get the machine parts which you are going to have to bring to the boss and put the parts in the right spots which I do not remember. Do note that one person can only hold a part for so long so regulate team carrying times while killing extras.Once parts are placed in the right spots the boss will start again and fall off the edge.