Originally Posted by Drip

Anemone is awesome!
Other piercers I like:
Daisy: she's innocent in a lewd manner and deals IMBA damage.
Snowdrop: One of the most "normal" girls available, but her debuff helps more than a Defense bonus against the tougher bosses. (A debuff squad is pretty strong overall)
Burning Bush: must-have for trigger-squads.
Mei Plumblossom: Together with Cattleya, she's a must-have on your whale-ship team. A guaranteed solarblast at the start, though I'd advice saving it against the smaller pests and help your squad get through those, instead of using it on the ancient bosses (it deals relatively very little damage against ancients)

Overall, I think all Pierce-rainbows are well-rounded units, so it's more a matter of personal tastes regarding their design and the strategies you use within your squads. If you don't play with a trigger-squad, then Burning Bush suddenly becomes a lot less interesting. If you don't use Solar Blasts, then Mei Plumblossom and Hollyhock become a lot less interesting..

Bear in mind however that solar blasts become a lot more useful in the future (they are definitely lackluster now).

As for ancient bosses, I'd like to offer another point of view, but it really depends on one question: does your main squad usually die before the end of the whaleship mission? If so, then I side with Drip: use the solar blast on the pests. If not, then save it for the ancient pest: use the blast after your whaleship makes both of its attacks but before the ancient pest starts their attack.

If done near the HP threshold such that the blast would push it over, the pest will flee after your blast without making its own attacks; with experience, you'll know where the thresholds are. The reason you want to use the blast after is if you exceed the threshold after one of the whaleship's attacks, then you don't get the benefit of the other one and using the blast first makes this scenario more likely which results in a DPT loss and puts your whaleship in a more vulnerable position towards the end of the map when the ancient pest will have more hp then could otherwise have been the case.

For my whaleship runs on 2-5 (895K before helpers and not counting the Hit-Type bonus), I usually use the blast after the whaleship makes its second round of attacks (this can vary depending and has sometimes ended up being around the 4th-5th round when the whaleship consistently used the side cannons instead of any specials). The pest flees, and the solar blast recharges from the smaller pest fights by the time I get to the ancient pest again. In a best case scenario when I could limit the first encounter to two rounds and each round was two specials with an overall total of at least two Isana Attacks, using a blast after the first two attacks is almost always enough to pass the second threshold before the pest gets to make any attacks. Blast recharges by the last ancient pest encounter, use it at will; the pest isn't fleeing anymore.

I realize this is a lot more involved, but the blast can still be a useful tool despite its relative low power.