I am absolutely positive of this answer, what you do is recruit Articuno, Zapdos and Moltres and then you talk to Alakazam in the Town Square. He will tell you to go to Legendary Island. The Legendary Birds will give you access to the Silver Trench.
at silver trench.
Lugia
99
I don't know find out yourself
Lugia is in silver trench 99f size four ****
after you recruit zapdos, articuno, and moltres.
Look for it in a water dungeon. I found a lot of them in the Silver Trench.
all you have to do is clear stormy sea and pitfall valley then be freinds with the three legendry birds then talk to alakazam then you gain acces to silver trench
You cant catch lugia on diamond and pearl but at red or blue rescue team in the silver trench
you have to have alot of one room orbs an a warp scarf
Silver Trench P.S. If I were you, I would bring at least 5 reviver seeds. What? that is how I defeated that cruel-99-floor dungeon
Go to silver trench 99F then throw him a blue gummy and then beat him up with your friend bow and then he will join he is in level 30 badly