Holiday Diet Fail is the second stage in Happy New Year?.
Battleground
The difficulty lies after you kill the peons, such as Doge, and B.B.Bunny. A lot of Hippoes will spawn after you reach the Enemy Base.
Strategy
The easiest way to conquer this level is to summon a lot of Dragon Cats. They have range, and a decent amount of HP. While you wait to spawn another Dragon Cat, use the Sexy Legs Cat, it also has good range.
If the player is rushing the level and has access to the right cats, Crazed Sexy Legs and Crazed Giraffe Cat can annihilate this stage in seconds.