"Warning. Unidentified zerg organism detected inbound. Largest on record." - The CP after detecting the Leviathan.
It attacks all ground units with the Spine Crawlers on its underbelly and shoots bile swarm projectiles at all the air units dealing massive damage very fast which makes it difficult to engage. It spawns at the top right corner of the map when you come close enough to the ramp leading to the base where it spawns and flies towards the top entrance of the Crap Patrol base. It hovers just out of reach of CP's anti-air turrets where it spawns Mutalisks and Brood Lords that will eventually overwhelm the defenses if left unchecked.
Unrelated to the Leviathan itself, the Hive in the top right corner of the map will be sending Primal Ultralisks to attack throughout the fight, which are a threat in themselves that must be addressed.
How to fight it Edit
You can use Hellfire Missile Batteries to deal damage to the boss since the boss wont move once he is near the CP base entrance. Place your missle turrets in the small gap next to the Plantery Fortress on the high ground (at the entrance) and place Flaming Betties and Gattling Turrets at the entrance. Focus fire the boss with the Hellfire turrets and keep killing Ultralisks as they attack you.
Stand on top of your ramp and Force Field the Ultralisks out, only attacking after there's been a wave of them and pulling back when you get low on shields. Thunderstorm will not be extremely useful on the Leviathan itself, but it can be used to deal with the Mutalisks.
You can continuously Snipe from a safe range with the right Snipe Improvements. It's also possible to Strafe to deal some damage and retreat to regenerate your shields while you wait for Strafe's cooldown. Strafe can also be used to thin out the Mutalisk groups.
With Adaptive Armor, Block Mastery, and Phalanx and some related Talents that upgrade Adaptive Armor, Leviathan can be facetanked. Irradiate also helps to kill the Mutalisks and Broodlings spawned by the Leviathan. The most important tactic you can use is to turn the Leviathan around, forcing it to face away from the CP base as it attacks you. This will allow CP Vikings to attack without the Leviathan returning fire, bringing it down much more quickly. However, because the Leviathan hits multiple times with every attack, if Adaptive Armor is not fully upgraded the incoming damage can stack up very quickly and potentially become lethal.
Stay on the ramp near the Bunkers and place Flaming Betties on top of it. Whenever you kill a wave of Primal Ultralisks and you have enough health, you can Stim over to the Leviathan and do some damage to it. When you drop low or see more Ultralisks, pull back, refresh your turret, and hold the line. It is very important that you only attack in between Ultralisk waves, as they can easily tear apart your base if you're not there to stop them.
The fact that the Leviathan remains stationary once it reaches the CP base makes it a perfect target for Siege Mode. Its range is long, but Long Range Combat Mastery and Weapons Research Mastery should out-range it. And if that isn't the route you built, you can still shoot down waves of incoming Ultralisks and Mutalisks to keep the Bunkers , Fortresses , and Missile Turrets alive.