To get riachi you must have defeated the elite 4 and got ruby and sapphire. You must have to gba's. Go to the direct room and go in trading. Do that with the ruby/sapphire. You'll have to go to trading.

So once you've done that you must have these in you firered pack.

Mew
Mew2
Articuno
Moltres
Zapdos
Porygon

Ruby/sapphire must have

Combusken
Any water
Oddish
Raichu
Azumarill
Marill

You must trade moltres for oddish

In the middle wehen there flying through you take the link cable out. Go back on and you'll have Riachi.