You think we can catch a Wailord and Relicanth, put them in a specific order, go through the hell that is Pacifidlog's current system, and do this?We must go for the regis: there is no other option.
First we will start with Regirock. His cave is in the south of the Desert so go there. When you get there, read the inscription and then go 2 Steps Left, 2 Steps down and then use Rock Smash. The Door Will Open and you can battle Regirock at the usual Lv. 40.
Secondly we'll continue with Regice. His cave is in Route 105 so go there. When you get there, read the inscription and then do a clockwise lap of the cave, staying next to the walls at all times. Once you have done a full lap, The Door Will Open and you can battle Regice at the usual Lv. 40.
And we'll end with Registeel. His cave is in Route 120 so go there. When you get there, read the inscription and then go to the middle of the room and use Flash. The Door Will Open and you can battle Registeel at the usual Lv. 40
I could see Regice being doable just because you have room for error without trolls. Maybe Registeel. Regirock is fucking nuts.