This mission is quite challenging. Be prepared to fight robots (if you have any weapon against them at this point in your campaign) and do that while running. The extra character who will be with you for this fight is invaluable. Using their abilities at the right time will be crucial to your success, especially Haywire Protocol and the AoE effect.
*** Spoiler - click to reveal ***Be advised that the fight is split in three levels and after the Derelict MECs start coming from the elevators, they will not stop. Although SPARK can take a lot of punishment take care to conserve its health for the rooftop fight, as its survival is a must and you cannot repair it. Shen, on the other hand can heal herself easily. Final battle notes: using Haywire on Julian is risky (as it has a bit more than 50% chance to work, but if it does you will control ALL MECs currently visible. Use them on Kamikaze attacks on Julian or on other MECs if you really must. Lastly, note that the generator turbines on the roof can be detonated. It is a good idea to blow them up when Julian is above them.