Here is what I did to get Jirachi.
First of all, you need to go to Wish Cave, but you will need the HM Surf.
I recommend being Alakazam to get through Wish Cave, because he has a high defense when he
is still level one. But before you go after Jirachi, you have to save Medicham which is on
the B20F. He will give you the Wish Stone.
After that, when I was Alakazam, I brought with me about 6 Reviver Seeds, 6 Oran Berries,
3 Max Elixers, 3 Huge Apples ( They are spred out all over in there. ), an Escape Orb, and
the Wish Stone. ( You only need the Wish Stone if you are not going to try to recruit him.
)
^^^^^^-WISHES-^^^^^^
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- When you bring the Wish Stone and you say "No" to Jirachi's wanting to join, he will ask
you something like...
"What wish do you want me to grant?"
The choices =
OPTION          WHAT IT MEANS
Lots of Money - All the money from Wish Cave.
Lots of Items - All of the useful items from Wish Cave.
A Friend Area - One of the friend areas you do not have yet.
More Strength - An item that will raise your stats.
Something Good! - A new pokemon will join your team, or your rescue rank will increase.
---Hint: If you choose "Something Good!" you will leave the dungeon and be woken up by
your partner the next day, who informs you of something great that has just
happened...your partner will not know it was Jirachi who did this.
Bad thing is, if you do not have the Wish Stone with you, he will not grant your wish...
However, if you decide you just want to recruit him, he will not grant you a wish...
If you happen to lose the Wish Stone, go to www.upokecenter.com and click on the "Mystery
Dungeon" link, and go to "Wondermail Generator." You will find out how it works when you
read the directions located above it.
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If you want to, instead of bringing 6 Reviver Seeds and all the other stuff, bring 20 Joy
Seeds...This works also, but I recommend bringing atleast 1 Reviver Seed, a Huge Apple, an
Escape Orb, and the Wish Stone.
If you would like to, bring any TMs your lead pokemon can use.
Say you decide to bring a partner...I recommend choosing Mantine, who can learn HM Surf.
And I say to choose Mantine also because it levels up fast, plus it gains 8 HP every
level.
******Rules to Enter This Dungeon******
1. You will lose all your money.
2. All pokemon on team will be reduced to level 1. ( It comes back after exiting. )
3. Know/Have HM Surf.