One thing that really helps me is, when making a center bar, I memorize which pieces need to be inserted. This is to not insert duplicate pieces and save moves. Also on big cubes like these you can usually find a bar that has about 1/3 pieces solved, so you should use those instead of making a bar from scratch. For edges, it's a bit hard to explain but I insert all of the edge pieces into the M slice (maximum 8 insertions) and make them all oriented. This is really efficient for solving.Try to make sense of these things by watching my solve videos, since I do it all the time. In general, just try to turn fast and do many solves.
