Before I start, this guide does not say the exact direction to go in, e.g. "Go up from
here, then two rooms left". It just points you in the general direction. Also, bring 10
Dusk Balls, 10 Timer Balls, and about 50 Ultra Ballls.

After banishing Team Galactic from their HQ, setting the 3 Lake Pokemon free and battling
Cyrus, and another Commander, head west from Hearthome City into Mt. Coronet. Make sure
you have Rock Smash, Strength, Surf, Rock Climb, and Defog. Navigate through this room up
the stairs, and continue on fighting grunts. Don't skip any, as the 3 hard battles you
will encounter on the top require your pokemon to be at least at level 45 for an easy
battle. Anyways, once you're outside in the snow, keep on using rock climb, getting
around, and try to capture a Bronzong. They're pretty good, but they're not required. So,
keep going until you find a slightly hidden cave. BEWARE: IT IS QUITE HARD TO FIND. Also,
if you find yourself in a room where there is a grunt blocking an opening in the center,
saying that he has no pokemon, you're headed the wrong way! Go back outside! Once you keep
mauling grunts and continuing through caves, you'll end up in a place called Spear Pillar.
You'll fight a 2 on 2 battle with your Rival and Commander Mars & Jupiter. He'll heal your
pokemon, then you'll battle Cyrus. He's actually (gasp) sort of hard. After sending him
away to lick his wounds, you'll be standing in front of an angry Dialga. SAVE BEFORE
MOVING! Now that you've saved, challenge Dialga and whittle it down to but a sliver of HP.
Paralyzing it or putting it to sleep helps. Keep on chucking Ultra and Dusk Balls, and
after about 25 turns, you can start considering Timer Balls. It only took me one ball: a
Dusk Ball, but it's a lot harder for most. Once you've obtained the mighty Dialga, you
will be escorted south, but instead of exiting, go up north past where Dialga was
standing. Pick up the Orb, and give it to Dialga. Use an Escape Rope once you exit Spear
Pillar, and you have gained access to Sunyshore! YAY! Also, I seem to have luck with
legendaries...in Fire Red, I accidentally (instead of attacking) threw a Poke Ball at
Zapdos(no, not an ultra or great ball, a POKE BALL) when it still had full HP and no
status problems. Guess what? I caught it...that really freaked me out. But, still, it was
cool! Well, hope I helped!