A new thread would probably work best. You'd need to provide sufficient details (more than just "a powerful character"). Suggest a level, class, role, and books available. If you have some ideas, then people are more likely to suggest something.
For instance: "Help me build an Archer" could be the title.
In the body of the post you could say that you are building a 5th level character and are allowed to use the Core books, the Complete books, PHB2 and Unearthed Arcana- maybe some other sources with the DMs permission. Items can be from the Magic Item Compendium or the DMG. Say you aren't sure if you should go Ranger or Fighter- what would people recommend? Is there any way of getting reliable sneak attacking with ranged weapons, and, if so, would a rogue based build work?
Something like that.
Likely someone will suggest a possibility, and someone else will try to prove that another alternative is better. Eventually the thread will deteriorate into a rules dispute, but you are likely to have a couple of good builds be posted in the meantime.
Good luck!
