The Leviathan continuously disappointed me with the damage output, but being on the most durable frame in the arsenal (Deathjack not included) it held up to more than one severe beating. The Leviathan is included in the list to cover against certain matchups, but bringing eighteen points worth of models with no assignments is a recipe for failure in every other game.
This weekend it killed a grand total of one model in four games, but it was instrumental to my success. During game one and four it took a Deathjack to the face and survived. Had it died I would have given it a fitting burial and cheered, since it gave me a shot at the most dangerous thing the enemy had on the table. An opposing heavy cannot ignore the damage from a Leviathan, pathetic as it may be, so at some point he will have to come for it. When he comes for it you exchange your Leviathan for his heavy, and Terminus profits when a threat to him leaves the table. I tried to have the Withershadow Combine in range to retaliate, but both times they failed to seal the deal (still a good setup).
The Leviathan was also used to shield Terminus after I learned my lesson in the first two games, since the effort needed to bring it down could also have taken down Terminus instead. In fifty point games two Leviathans can actually block LoS to Terminus, which is an incredible boon in some matchups like Menoth lists featuring the Avatar. If you're facing the Avatar and his gaze is making you stay for a messy death the following turn, you load up both Leviathans to full and just walk in front of him to break LoS. Not only will the Avatar take quite a bit of damage (around 8 points from each Leviathan, assuming you can shoot him of course) but Terminus can just walk away and live to fight again. You should remember two rules when doing this...
- The Leviathan is steady, so there's no way to knock it down in order to get LoS to Terminus.
- Even when slammed the worst thing that can happen is Terminus getting some downtime, since the Leviathan is staying where it is. Mind the throws though, as having the Leviathan thrown out of position could be very bad news.
In summary: The Leviathans are there to force action from an opponent, and take the beatings that could take down Terminus instead. When thinking about it like that, the cost suddenly doesn't seem so bad.