that's right, there are few bosses that one defeats and they don't even put a fight the next time that you fight them, luckily, there are some that still give some fight, thos are
shadow queen
bowser (PM)
the master
chapter 6 boss from PM (don't know his name)
cristal king
culex
smithy
mother demon
galacta knight
blaze
dark kahn
etc
On-topic: The Basilisk from Harry Potter and the Chamber of Secrets for the GBC was really difficult, because 95% of your sword attacks wouldn't do any damage; meanwhile, all of the Basilisk's attacks on you do a minimum of...oh, say...80 damage. So, you have to rely on a shitload of HP (called SP in this game, for Stamina Power) boosting potions and really good card combo attacks in order to survive.
i dont know about Shadow Queen. i mean when i first battled it i was Lv. 17 and then i battled it yersterday (after going through all the chapters from the start a few weeks ago) on Lv.28 and to tell the truth i had 35 HP and 5 FP left, all my partners were fully powered and i used Bob-Ombast alot, aswell as Sweet Treat. Vivian was the only one who died and that made me vulnerable to her 14 damage attack. but i thought she was REALLY Hard but that was on Lv.17 and on Lv.28 i kicked her ass by a long shot.
Cackletta's Soul is challenging, but to be honest I don't get to the final boss very often in a game, so I don't really know. Out of the ones I've gotten to, probably Cackletta's Soul or Shadow Queen, both of who I have never beaten.