I'm aware this strat is not as fast as WR strats but it's a very good and consistent strat for people who are less skilled. When Luigi has best posture (when he's standing straight up) hold the B button while sucking up the ghost, when Luigi starts to bend over and get dragged, start mashing the B button while still pumping the ghost until Luigi goes back to best posture and when he does that start holding the B button again. Start mashing it again each time you bend over. Repeat this process til you finishing off the ghost

Here's a video of me doing it on Bogmire for example: