War Phantom
The first boss battle in Ar tonelico: Melody of Elemia, these enemies also serve as a sort-of preview of the more advanced Beast-type enemies you will encounter later on in the game, like the Night Stalkers and Mist Walkers. They only have one attack: Bite, which causes low physical damage to a single target, but what makes them an annoyance is the fact that they are immune to physical attacks, and since you won't have a Reyvateil at this point, this reduces your options of attacking them to just using the Missiles you should have Grathmelded prior to battling them.
These monsters are immune to Wind-elemental attacks, so Lyner's Impulse skill is useless against them, so if you happen to run out of Missiles, put Lyner in healing duty and leave all of the attacking to Jack. However, these monsters are pretty weak, so using just a few Missiles or a few attacks from Jack should be enough to defeat them.