if it were me, i'd go for the 2 light ones and triton OR balor.. next event is light, but you only use tsuku in your sub team right? your dark team is the best, so no need to focus on her for now.. her ability to blind allies is annoying too, but since you have balor no need to use it though.. still, after the light one we'll have fire and then dark, so triton can be nice for your water team, her ability can be increased with the max star though i'm not sure which ability it is.. and for your light team, you said it's not your best team.. so better to try to make those partners stronger and higher lvl... who knows, this event still going on for a few more days so you can get more books but.. i'd leave tsuku for last, definitely limit break balor before tsuku...
then again my advice probably sucks...cause i'm not too good at this game