"We've recenlty noticed an increased level of Alliance activity at the old corporate headquarters site."
"Our declared time of attack is quickly approaching. The enemy is probably looking to execute soem sort of last-ditch plan."
"We want you to launch a preemptive strike against the forces assembling at the facility."
"This mission carries a great deal of risk, the exact strength and composition of the enemy force is not known. Please prepare accordingly."
The first part of this mission is to deal with the Type 86 vehicles that appear. It is best to use something like the ELF3 to take out the vehicles quickly because they appear ad infinium and can deal a lot of damage. Eventually the Alliance officers begin to talk and right after one says "Deploy the Leviathan" destroy the nearest Type 86; they won't respawn after this. After the Leviathan appears, destroy the remaining Type 86s because they can become annoying while you try and take down the Leviathan.
The Leviathan that is in this mission is stronger than the Trio featured in the Leviathan Path. It attacks with twin Grenade and Laser Cannons along with deploying large amounts of missiles. Its ECM feature is also stronger so having a head like the STING or DRONE coupled with a radar and or the KANJI Optional Part are very useful. It is weak against missiles much like the Pulverizers but it's figure eight flight pattern and ECM makes it hard for a solid lock. Something like the dual back mounted missile launchers with extensions or micros are useful because they don't need a long time to get all missiles to lock on. Fast lock FCS models are also a must because of the ECM. Keep behind cover either behind the central building or the outer edges to avoid this machine's powers assault. Keep up the offense and it will fall. Another option is to just throw large rockets at it until it dies. It's huge and incredibly easy to hit, even for someone who is inexperienced with rockets. Another DANGEROUS way to destroy it. Build an AC with moderate defense and speed, also equip it with a micro missile launcher with FUNI extensions. Use it against it, and it will die quickly. Make sure you can dodge in the air or else it will decimate you with it's grenade launchers. For the missile launcher, the KARURA is recommended. If you do it correctly, you will be left with 28 missiles.