yes, stark mountain holds a sidequest and in my opinion a pretty difficult one too so be prepared. catching roaming legendaries will also increase the levels and some other stuff aswell but i forgot those sadly, it might be an idea if you browsed around here
hnm hm, no problem.
oh, and seeing as you go into caves to catch the lake trio try to use dusk balls instead, they have an even higher catchrate then an ultraball, do know that they only work in caves and at night.
go to the chateau where rotom is and save in the main hall of the mansion and then turn of the game, go to the option menu of your DS (lite) (i) and change your clock to 23:59 which is one minute before midnight.
switch on the game again, wait for one minute untill the clock on the poketch hits 00:00 and go to the TV and have fun catching rotom
it's a trick i discovered by accident years ago with D/P, timechanges in the menu will take effect at midnight and rotom is catchable after 21:00.
hmm, can you give me a list of which ones you are after?
here is a list of the sinnoh pokedex, click me
just open your ingame pokedex and look for the empty spots and use the numbers to determine which ones you need in the list i gave you, the routes are described in it but maybe i have some other possible solutions.