The debuffing queen of PVE, Leviathan is a card that started mediocre and has quickly jumped to a powerful card. With the rapid changes and deployment of buffs and bosses that focus around the use of them, Leviathan's ability to completely remove positive buffs is a useful, if not reliable way to change the tide of a battle. Couple this with a 30 cost stat total (The highest in the game even), and Leviathan is a solid and useful card to have around.
Leviathan's power in PVE lies heavily in her ability to remove buffs from the enemy, coupled with new dungeons that have bosses revolving around their buffs (Such as Chen's slowly increasing attack power with his agility gain, and Senju's ability to flash +5 either attack or defense). A proc will neuter some of these and greatly hinder others- however, the rate seems to have iffy chances. However, her attack also is a typical 3-hit, meaning she can still output solid damage on red targets and claw-weak targets. In PVP, she is the only solid blue claw attacker in the game at the moment, only sharing the field with Nadja, Stingray_X, and Lao_Hu, two of which are questionable and one that does not work in PVP very effectively.
The downsides however, are there- Her subskill is almost completely useless. Her actual attacking total is the weakest of all the Archdemons, and is in the lower range for 1400+ attackers- and in PVP, while her damage is still very solid, her sub-effect will have little use. She is also arguably contested in PVE by Beelzebub, who does what she does with the addition of stealing the buff.
Overall, Leviathan is perhaps the weakest of the Archdemon still, but still a very powerful and useful card to have around.
As said, Leviathan's skill will absolutely remove -all- positive buffs on the enemy, however, the rate is 50% as far as we know, and is only on Skill Activation, not per hit. Otherwise, Leviathan's skill is exactly the same as most other 3-hit attackers.
Leviathan's skill is always at it's strongest versus bosses that love to buff or have gimmicks revolving around buffs, but the distinction between when she and Beelzebub are better exists. Specifically, bosses that only buff ONE stat are Beelzebub's primary prey, as she'll do the same as levi and also give you the buff. When more buffs are in play, Levi takes the advantage as she will wipe out all of the positive buffs and leave the boss at status quo. Packing both of them however, is never a bad idea when faced with a boss with pretty scary buff numbers.
Levi's subskill has a host of issues, notably, the chance to proc it is estimated as 25% at the moment, and it only adds a +1 to either attack or defense for your trouble. While non-girl a sizable pool, the cards that gain AOE or multiple procs on allies are limited, and only three cards are claw-based attackers, one of which is a card reserved for PVP (Don_Koala) and the other who revolves around Agi buffs (Lao Hu), which levi does not buff and the final is Stringray X.
While a mediocre stinger for a Pigeon Priest or perhaps a Whale or Baphomet with nothing better to slot, Levi's sub-skill is almost completely useless, especially with much better oftens for buffs existing (such as her competitor, Beelzebub)