If you’re talking about breeding 5/6 IV Pokémon, then a 6 IV Petilil is not needed at all. You need a 6 IV Ditto and a Destiny Knot.
If you’re talking about simply breeding more Petilils, then you don’t really need a 6 IV Petilil, (although one is preferred) you just need a female Petilil and another Pokemon in the Grass group (or Ditto) and you’ll get another Petilil.
And if you’re talking about breeding Pokémon in general (not just Petilil), breeding any Pokémon (other than legendaries and Ditto) with a Ditto will get you the Pokemon which you bred with the Ditto.
More info on breeding for IVs can be found here.
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