It would be a good idea to combine shadow and celest heat in one angel heart since celest heart are really a rare drop (quite inexistant) and shadow a quite reasonable drop (at least with devotion 1 per day but 220 devotion is too greedy to have it).
I just augment celest to rank 8 but only have 18 celest heart for so long and 55 are needed so it will be impossible to upgrade her (except paying 2400 gems :/)