honestly, do as much as you can, even if the fight is hopeless.... there's a few reasons why:
-dying does not prevent you from getting rewards if the boss is killed
-help might show up later on and you get up to 24 hours to kill the boss anyway (when the day resets, you're out of time and the levels and grails reset)
-you still gain PP based on what you do (this is a BIG reason to fight anyway unless you got all the individual and union PP rewards).
so if you hit a "point of no return", just jump into each possible raid until you die in all of them, then stick to lilim farming until a new raid pops up.
i think this let's you into all 6 possible union raids and a lilim one.... unless it's like the normal ones where you only get in about 3 or 4 max before one of them has to die or time out
considering this is a newer player, i'd honestly use Sol over kikuri since they won't be able to take as much damage to really get the most of of the "kill or be killed" strategy.
that usually involves a lot of power and that won't happen for a while (they'll need the assault and/or high atk stat as in 30k+ to support that)
plus awakened Sol should be their priority so they should also get as many dragonic eye shards and such as they can (they should be getting materials as much as possible too). Awakened Sol pretty much rips the game a new one until we get later content, even then she's still one of the strongest kami in the game