See, there's this Rage FAQ at GameFAQs.
On second thought, there isn't a Rage FAQ at GameFAQs, and that needs to change

After discussing some stuff with assassin I tried to assemble a list of bosses and the ultimate Rage in those fights. I discovered very quickly this is a matter of preference. So I thought this should be open for interpretation thus be decided by more than just one person. For instance, what's best for Atma? A Merton/Flame Shield combo? NightShade, but a fire weakness? Magic Pot, but the terrible amounts of time that will take in, for instance, a SCC?
So here's five boss moster formations in alphabetical order:
Air Force, Laser Gun, MissileBay, Speck
Atma
AtmaWeapon
Blue Drgn
Chadarnook, Chadarnook
I'm personally tempted to go for:
Rhinox (death protection against Missile and Launcher, lightning-absorbtion for Tek Laser, Diffuser and WaveCannon and physical attacks, remains only Atomic Ray to worry about, and Speck when the Evade bugfix patch is applied).
NightShade, as Magic Pot will eventually have to deal with Ultima. A hard one as most Rages could apply with equipment changes.
Bomb, as it has the fire-absorbtion going for it, together more status protection than Grenade, weird as that may sound. Or Chimera when fighting along Edgar, for that %25 chance of boosting Aqua Rake in power.
Aspik, water absorbtion, Giga Volt.
White Drgn, for death protection, Starlet form healing and Chadarnook from hurting.
If we're half-way done with these we can discuss some more.