In Pokemon Alpha Sapphire I caught Mesprit, encountered Azelf, but I haven't seen or caught Uxie. I have seven gym badges, and I want to get Lugia. Can somebody tell me how to get Lugia without having it take several hours?
After catching or defeating Kyogre, head to Sea Mauville with a pokemon in your party that knows Dive and find the Scanner in the submerged control room. Once you have the Scanner, head to Slateport Harbor and give it to Captain Stern - who will give you the Tidal Bell in exchange.
With the Tidal Bell, return to the submerged control room in Sea Mauville to encounter a portal thing - which you interact with to encounter Lugia. (The process to get Ho-oh in Omega Ruby is the mostly the same, but the portal is on the deck)
See also this Youtube video for a quick visual walk-through of the process.