During the battle however, don't worry too much about your insanity. If you're all out when the zombies are summoned (in any stage), then just kill them before they see you.
As for the second stage of the Guardian, this is what I do:
Cast a 5-point shield barrier thing (you know, that wall barrier that protects as long as you stay inside it) RIGHT IN FRONT OF THE GURADIAN (This doesn't protect you from the Guradian, but from the zombies).
When he summons the zombies, stay inside the barrier until they all appear. Notice how if you stay in it, they don't do anything? OK. Just step out of the barrier, then get back in. The zombies will have seen you, you lose some sanity and they come toward you.
Make sure you have a 5-point magic attack on a button handy, cause as SOON as you step back in, cast the spell. By this time, the Guradian would have started glowing, so when you finish the spell, it hits the zombies and the Guardian.
The shield is there to protect you in the case that the zombies get to you before you finish the spell, whereby the spell would have been broken and you would have lost magic for nothing.
Now, make sure to note that all this would have been done in the space of a second or two (assuming you already have the magic barrier shield up).
So keep doing this until the third and final stage!