It doesn't really have to do with the difficulty. (I don't think Perseid is nearly as hard as the last two, though.) The thing is that Fairy Destruction Ray will inevitably reduce your motivation to 0%, even if you went in with something like 300% or more. That means that bombing to an earlier spell does nothing but cost you a bomb, since the motivation you saved is lost anyway. So unless those cards will literally end the run if they hit you, you shouldn't bomb them.
In fact, since dying charges your bomb gauge slightly, it's actually better to die to lose the motivation to the beam. So you should suicide for bombs if you reach the final with any extra motivation. The last phase can be done with a single bomb, not needing to dodge anything. The second phase would probably take two bombs to completely trivialize, though even one bomb goes a long way. The first phase should be fairly trivial anyway. So having two bombs makes it much easier. Having three basically nullifies its existence.
So basically, reach the last nonspell with lots of motivation. If you stop bombing from there, you should have three bombs for the final spell, assuming Perseid and Blazing Star don't completely wreck you.
e: Better yet, as long you have over 100% motivation on the final spell, just stay underneath the orbs at any cost, choosing to die rather than go into the beam. That's better then outright suicides, I guess.
