eh invincible monsters never learn how to deal with modern weaponry
You know, that is kind of a theme of the show. How much trouble many monsters and demons have adapting. I still love Trick's "it's called an uzi, it would have saved your life right now" line. The most dangerous enemies are usually those that can adapt with the times.
I can see you issue with Grad day as it isn't a super epic battle type finale, its meant to be more of a fun and exciting end to a season that has really tried to mix fun and exciting (see: the Mayor's dualistic personality). Its another ep where expectation and mindset can really affect enjoyment.
I watch something like Farscape for big dramatic epic finale's, I don't tend to expect more than a fun episode with a big battle from Buffy.
And as a warning (not really spoilers don't worry) try not to go in with any expectations for the season 4 finale. Just watch and absorb.
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"Hope is allowed to be stupid, unwise, and naive." ~Sir Chris