Nil Vata is a boss in Final Fantasy Dimensions. He is fought at the Dark Gate. Vata fuses himself with Dark Flow to make it become tangible so that the warriors can damage it. When Vata completely loses control, which is when he reaches the second form, the fight will be much harder.
Nil Vata has two forms. At the first form, he only uses Dark Flow's attacks, which are Comet, Flare and Gravity, and physical attack. Vata is slow at this stage and his abilities can't damage more than one character. However, when he loses control of himself, he gets into the second form and starts using Vata's familiar moves: Buffet, Jump and Wild Dance. Buffet deals Wind-elemental damage to all targets and may cause Stun. Jump deals massive physical damage to one target and Wild Dance will is a five-hit strike. He can also double-cast Gravity.
At the first form, the party doesn't have to be healed a lot as Nil Vata's attack only target one member at a time. The player can combine Mighty Wall with Hastega. The party must attack him consecutively and deal as much damage as possible. Final Heaven, Midareyuki, Phantom Rush and double-casting Meteor are the way to go. Thing will get nasty when he gets to the second form, and it is best if he didn't have a lot of HP when that happens.
At the second form, Nil Vata is considerably stronger. His attacks deal a heap of damage and healers may not be able to keep up with it. He should have received lots of damage already, and thus the player should finish him as quick as possible.
Nil is a word commonly used to mean nothing or zero; it is one of several names for the number 0.