Uh...I don't think he's immune to Toxic Ball, but I could be mistaken.
Regarding that Renzina demon fight, if you're not at level 100 and haven't spent a lot of coins on your characters, you're going to struggle. Also bear in mind that you should be strategic about it...get your active character below 100HP without it getting killed, and you can opt to switch it out. The most dangerous character to do this with is Kelly because her defense is so awful, but if you're careful you can still do it. The idea is to pelt her with scrolls till your character is at 100HP or lower (or one hit from dead), then switch out. You should be using Death Dimension with Kelly and Final Oblivion with Simon. Using Mass Slaughter with Cecil and Murder on Demand with Shauna wouldn't hurt either. Ross can stick to Destruction of the Heavens if you have trouble with Meteor Rush, and Gabriel can use whatever you're comfortable with. If you do this, you should be able to win.
Regarding Pierre, don't yank anyone from battle immediately. Even if Gabriel gets the call to use Aura Master, probably the weakest overall damage you can do to Pierre, that's still one round of small damage and one round on which your other characters are not being hurt.
As far as galactic dusts go, Bryan, did you do the second floor of Tower of Fear? If not, go do that, then revisit all the save points.
Regarding the Protector, I'm quite sure I'm right. The item in question is hidden though, so start mashing Enter when you hit the top wall and move from one side to the other. You'll find it.
To be clear, I'm talking about the area where they said there was a cave-in.
Regarding Final Shot: get your level up. Higher level means more HP and higher defense, which translates to more rounds to screw up. Don't worry, you'll figure it out.
Check out my Manamon text walkthrough at the following link:
https://www.dropbox.com/s/z8ls3rc3f4mkb … n.txt?dl=1