Okay, so I don't believe there are sub natures, or at least according to my knowledge of Pokemon, which is to the extent of a halfway decent competitive player. Perhaps you mean IVs, in which you mean the hidden value in a stat that ranges from 0-31. In the case you do mean that, I would recommend getting someone to clone a 6 IV Ditto for you and have it hold a Destiny Knot to refine your Chanseys, and every so often cycle through your highest number of 31 IV stat'd Chanseys to put the one with the most perfect IVs into the daycare to breed with Ditto to eventually get a flawless competitive Chansey.
To answer your question on nature, most people use Bold or Calm natures on Chanseys / Blisseys because they can wall certain threats in the metagame or in-game. But if you're going in-game, I'd recommend using a Modest nature if you intend to not make it a wall, as you can get a little more fire power out of it.
Hope this helped, and if I wasn't clear, or you're confused, or I was confused, don't hesitate to explain a bit more in detail or ask me to explain in more detail!
Source(s): Personal knowledge, Breeding in XY, competitive battling knowledge