Thank for the answer you made anon (and the completion made later by the other anon (or the same as far as i can know :x))

To obtain KH on "regular" basis, as far as I know, is either stack up jewel with completion reward on quests and login reward and spend them 3.000 at a time and hope or you may also use gem gatcha even if you will mostly end up with R KH (whith is not that bad for F2P player I guess).

If there is another solution, I would be glad to know ! (and for elixirs too)

(I hope I don't bother with my poor english, as you may notice, it's not my mother tongue)