easy. you go up to the foe and use it. if you want to use it on a boss like gengar or zapdos, you can't.
here are other items or moves not able to be used...
rock smash
blast seeds------------- they are really weak, but usable.
any orbs
hms
dive-- unless you are on water
and, if you are on water, dig.
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You find it randomly on dungeons with a lot of floors
Become a goast pokemon, use a mobile orb or get the IQ skill super mobile. You can also use a mobile scarf.
No.
there is a place within guilded hall, if you stay in far enough you will reach the golden chamber where thundurus and tornadus will appear. I recommend using gifts to reqruit them, or use a monster orb to make a monster house.
The Griseous Orb is only available in Pokemon Platinum.