You can't use AR. .
How To Catch Regigigas Without Regirock Regice Registeel?
Ar
There is no possible way unless you hack it but if you have Platinum you can and regigigas is in snow point temple
Action replay or the 3 pokemon
Use a action replay ds or dsi
If you don't hack the game, then the only way to awaken him is if you have those three pokemon in your party
Look the only way to catch him without the three regis(regirock registeel and regiice)is to hack the game with cheatcodes from the action replay!Hope this helps!:)