It means you must use Lancelot's skin. so you can use her with Gawain, Arthur or Charl's skills, but it won't count if you use those other three.

Also, the "without withdrawing" is a super new feature. Similar to Tower, you can now go back to a quest once you die to finish it off and complete certain missions, but for some you have to complete them without dying. Similar to using an elixer, I suppose.