Finally got 600 kills, had to take advantage of shrine blessing though.
Now I have Joy... and frankly, I'm not sure who to use it on. I don't know how much EXP it takes to level Sybilla, for instance, from 86 to 99, but I'm pretty sure it's less than the amount Joy gives.
EDIT: Looking at
this table, 150K EXP will put
any unit from level 1 to
max level. So the best bet is to Awaken a Black unit, then use Joy on her to max her out. At least I have a few candidates... do I do so with Karma, Nag, Despera, or wait for some other Black unit?