Regigigas' Slow Start is actually useless, but you can cancel this ability:
Like Slaking, Regigigas is only useful in 2 vs 2 battles.
Simple Beam might be better to use. It's possible that Slow Start is meant to give the opponent a chance considering Regigigas' very high Attack and other stats.
I've also read that without the ability in spite of its stats, the stats affected by the ability just remain, meaning Regigigas was meant to definitely be weaker and slower until the ability triggers the change.
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Let's do this by base stats to avoid the problem of determining a moveset. I am taking Regigigas' ability Slow Start into account. HP: Even, both have 100. Attack: Dialga has 120; Regigigas has 55 (after Slow Start; 160 normally.) Defense: Dialga has 120; Regigigas has 110. Special Attack: Dialga has 150; Regigigas has 80. Special Defense: Regigigas has 110; Dialga has 100. Speed: Dialga has 90; Regigigas has approximately 35-45 (after Slow Start; 100 normally.) I didn't bother to calculate Regigigas' speed exactly because it's fairly obvious that Dialga is faster. With 4 stats higher and 1 stat tied with Regigigas, Dialga is clearly better - and Regigigas' 10 point special defense difference isn't likely to have any effect.
Regigigas is a Sinnoh pokemon. Not a Hoenn.
Regigigas is a pure Normal-type Pokemon.
You cannot get Regigigas in Pokémon SoulSilver, you will have to trade it from Diamond, Pearl or Platinum.
From what I have heard, Regigigas is a Hoenn pokemon. You will have to trade it from Platnium. Sorry!