You get Regice for Saphire, Ruby, or Emerald with a lv 50 Relicanth and a lv 50 Wailord and go to an underground place. Or u can just use hte action replay.
You have to have regiice, registeel and regirock to be able to fight him
you catch regirock by going to mount cornet with a lv. 100 shiny regigigas.but i forgot how to catch regice.
regiice cant be caught in Pokemon diamond, it can in platinum version, how ever you can transfer a regiice from ruby, sapphire or emerald if you have 1, or there is the method of cheating in which i am against. cheats lead to glitches and unsolvable problems. u can only get the regi known as REGIGIGAS.