I'm looking to introduce an recurring enemy in my new 4e/Ultramodern4 campaign that's an on-going, ruthless opponent. I want this specific opponent to be an exception of the normal 4e pattern of all enemies being reasonable fights; I want to make the enemy overpowered enough to strongly encourage players to deal with him in a non-direct manner without removing it from the table entirely, or artificially limit it (by physically removing access, or some sort of ritual to allow the enemy to escape, etc).
With this in mind, I want to level him outside the normal leveling scheme.
Since the highest official monster level is 36 (and the highest official player level is 30) it seems the answer might be 6, but this is just an educated guess at best.
Mathematically, how much of a level difference can I make a boss compared to the party while still allowing the possibility of direct confrontation?
Note: Most of the party have put considerable effort into optimizing their characters, if that might play in at all.