You're (almost certainly) not stupid. Especially for this mechanic it can be really tricky to notice what the special condition for dealing with it is.
The walkthrough that comes with the game gives a hint (not a guide or strategy, just one hint) for each of the superbosses in the game, so I recommend using that if you're still stumped. For me at least, it helped give me a nudge in the right direction while still leaving me to figure out the fight itself, so I didn't feel any loss of achievement.
Just use the bookmarks or skip straight to the last page of the document so you don't have to see any unwanted spoilers, since it's very exhaustive regarding any topic that isn't the superbosses!