Giratina-O was no harder for me to catch then Dialga(Diamond) or Palkia(Pearl) & definitely easier then catching Suicune or the Legendary Bird that is not your mascot in HGSS...
I agree with GofD though, that Regigigas was so difficult for me to catch even with False Swipe & the ability to put it to sleep that I just gave up on getting one with any kind of good IVs or even a good Nature, that's how bad it was...
It took me 2 tries to catch Giratina-O & that was only because I wasn't probably prepared to catch it aside from having the right Poke'Ball for the job(not the Master Ball of course)...
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