Agree that the nature needs adjusting, but Pressure is also a viable Lugia ability. Lugia can actually stall out some weaker attacks, some would-be ones like Groudon's Stone Edge thanks to Roost and its higher speed.Lugia likes his Multiscale and doesn't really need a blue pentagon, since it can't even participate in any metagame where the pentagon is required. Don't know about the nature, but personally I don't really see the trade value here...
Not sure if Multiscale's preferred, but Pressure is by no means an unviable ability.