Regigigas has the ability "Slow Start". When he is sent out, his Attack and Speed are both halved for 5 turns (his normal base Attack and Speed are 160 and 100 respectively, so after that they become basically 80 and 50).
As such, you need to try and play around this flaw. You can either have a partner use Skill Swap to take the ability off him and let him fight at full power right away, or, you can have the partner use Trick Room to let Gigas hit first, but at cut power. (Plus, it trolls your opponents.)
Those are the two most common used. I chose a Skill Swap strategy, and although I went 3-4 that day, it worked for the most part, and did exactly what I needed it to do.