After reading one of the designers for Dungeons and Dragon's 4th Edition on how to deal with solo monsters, its just one fresh idea after another!

http://community.wizards.com/pluisjen/blog/2009/11/14/a_new_look_at_solo_creatures

Create the monster in parts, increase its actions pending on parts and number of party, and spread out its hit points. This would make it interesting for if a monster is spread out in its action, would that mean each part gets an initiative? A party of 5 versus a red dragon, a LONE red dragon whose each of its part: head, tail, arms, wings and body have its own initiative? Would be nice to ready an action such as the tail prepares to swipe at a character when it comes too close to its rear end. This would also solve the issue of control actions such as daze, stunned, cannot make attack actions, etc. if it attacks a certain part. Funny, I was reading up on Hydras and those suckers have individual actions; maybe they got their ideas from this monster?

The only drawback is the hit point upkeep; but then again, you're treating the whole solo monster as a gestalt.

Another idea is when a DM has prepared an adventure to the number of gamers (ie. 4 gamers show up for a 5 person level 10 encounter), the modification would be reduce the hit points of the monster. That would probably 20% hit point reduction for ALL of the monsters, otherwise leaving with monsters at 80% of their hit point total. I should try this out, the challenge is there its just the monster's hit points is reduced.