easiest way would be to have a Pokemon learn flash that you obtained at sprout tower. also u can get through with out flash as long as you have some repels because it'll get boring of having wild Pokemon attack every 10 secs.
In Pokemon
the dark cave is near the sairs to vilot city
You can only come from the top down through dark cave, so you cannot go up from route 2 even if you have flash
the dark cave can be entered on route 45 or route 46
Dark Cave
Dark Cave
There are no legendary in dark cave in soulsilver that i know of, but i know alot of legendaries and I could list some of them if you want.
In Dark Cave.
in dark cave
Dark cave.
just look the walkthrough on youtube
The only dark type Pokemon you can get in firered is murkrow which happens to be in lost cave on five island.