I currently hold the world record for fastest time to solve a 40CM 3x3, while a lot of this is due to not many people wanting such a ridiculously sized cube, there are some techniques that I have developed which have allowed me to turn cubes like this very fast. Essentially, it is a bit of a workout and me being quite tall probably helps a bit, my arms can easily stretch all around the cube so I never need to rotate, this should be the goal with giant cube solving, you need to try and eliminate rotations, it is even more important than with regular cube solving. While it can seem good to hold the cube against the edge to turn more easily or rotate so that every move is a U move, the fastest way of doing it is probably to force every move with brute force and just try and ensure layers line up properly, maybe magnetizing the cube will make that easier but I haven't tried that. Obviously F2L efficiency is very important here as you can't spam TPS as easily, make sure that you can insert pairs in from all angles. A lot of people mess up during OLL and PLL as algorithms are ingrained in their muscle memory and that doesn't transfer to a giant cube where every turn requires your whole hand, so it is important to know your algorithms really well.