C0 is enough unless you want to play Geo teams (then need C2). Better result if you save the cons for Mavuika or Citlali.
If your team have 2 or more PECH (Pyro, Hydro, Electro, and Cryo) teammates, Xilo will give 36% res shred for 15secs after E + 2 normal attacks.
If the team only 1 or less PECH, then there is no res shred. Only 30% dmg bonus to normal and plunge atk.
She also give an extra nightsoul burst in additional to the normal nightsoul burst. Useful for Natlan characters.
Her E+2NA will consume 90 nightsoul points instantly = 90 fighting spirit (C0 Mavuika) or 112.5 fighting spirit (C1 Mavuika). 100 fighting spirit to burst, 200 fighting spirit max.