How do I get through Team Magma's 2nd hideout in route 129 (or something..

I assume you are reffering to the Seafloor Cavern. There are actually several ways to get through this cave, but this one is the simplest. Once you surface inside head north through the entrance and take a single pace to the west. Head north again untill you reach a cracked stone, then smash it and roll the boulder south of it from in front of the passageway. Travel east and roll the other boulder out of the way, then head north, east, north, east, and north so that you end up in a room with several passages which have a boulder wherever they intersect. Push the first one north, then follow the path on the eastern side up to another boulder. Roll this one aside, then head west untill you hit the next boulder. Shift it and smash the previously adjoining rock, then head north to the next boulder. Move it aside, then proceed west, smash the rock in your path, push the boulder aside and head north into the next room. Surf north into the first current, then continue east, north or east, and west through the currents. Once free of all moving water, head north and through the passageway int o the next room. Travel east and north untill you reach a boulder, then shift it aside and take one pace east and one pace north. Push aside the boulder to your west, then occupy its previous position and head north untill you push aside another boulder. Head east and shift the boulder in your path, then take a pace north and shove east untill you have enough room to proceed northwards. Go north into the next room, then travel west around the ledge and push the boulder just west of the southernmost central one north once without occupying its previous location. Take two paces east and push the opposite boulder north as well, then push the centeral boulder east and continue north untill you occupy the original position of the northernmost centeral boulder. move through the boulder to your west, then push the boulder west of the northernmost central boulder north from its original location. repeat the process in reverse on the eastern side of the northernmost central boulder, then push said boulder east and continue north. From here head east, then descend the flights of stairs and follow the curve west, south and east untill you reach a pool of magma which has groudon in its center. Stand as close as can to the magma due south of groudon, and you will be challenged by Maxie. Once you defeat him, you will recieve all of the information neccessary to proceed further.

I assume you are refering to the Seafloor Cavern. There are actually several ways to get through this cave, but this one is the simplest that I know of. Once you surface inside head north through the entrance and take a single pace to the west. Head north again untill you reach a cracked stone, then smash it and roll the boulder south of it from in front of the passageway. Travel east and roll the other boulder out of the way, then head north, east, north, east, and north so that you end up in a room with several passages which have a boulder wherever they intersect. Push the first one north, then follow the path on the eastern side up to another boulder. Roll this one aside, then head west untill you hit the next boulder. Shift it and smash the previously adjoining rock, then head north to the next boulder. Move it aside, then proceed west, smash the rock in your path, push the boulder aside and head north into the next room. Surf north into the first current, then continue east, north or east, and west through the currents. Once free of all moving water, head north and through the passageway int o the next room. Travel east and north untill you reach a boulder, then shift it aside and take one pace east and one pace north. Push aside the boulder to your west, then occupy its previous position and head north untill you push aside another boulder. Head east and shift the boulder in your path, then take a pace north and shove east untill you have enough room to proceed northwards. Go north into the next room, then travel west around the ledge and push the boulder just west of the southernmost central one north once without occupying its previous location. Take two paces east and push the opposite boulder north as well, then push the centeral boulder east and continue north untill you occupy the original position of the northernmost centeral boulder. move through the boulder to your west, then push the boulder west of the northernmost central boulder north from its original location. repeat the process in reverse on the eastern side of the northernmost central boulder, then push said boulder east and continue north. From here head east, then descend the flights of stairs and follow the curve west, south and east untill you reach a pool of magma which has groudon in its center. Stand as close as can to the magma due south of groudon, and you will be challenged by Maxie. Once you defeat him, you will recieve all of the information neccessary to proceed further.