Actually, Bold is a good nature for Lugia. Augmenting its defense will only augment its ability to perform as a physical wall, which is all the way nice. If you have Multiscale, you are pulse rocketing this wall ability to the extreme. However, don't be disappointed if your Lugia is Pressure Lugia because Pressure allows the flying bird to perform a PP Stall, which is excellent to know and to do. Do not change it. Impish and Timid are good natures, as the former allows you to use Dragon Tail better while the latter allows Lugia to function as a special machine, on the both sides (specially defensive and offensive) after Calm Mind.