Well the party did it. Threw 2 encounters at them tonight, both a solid eight threat levels above the party. They took them both down, yes it was a fight, but it wasn't as close to a TPK as I would have expected.
For clarity sake, I am running the Henchman. I know, there is serious objection over this sort of thing, so let me clarify as to why I think it is okay in this instance.
Firstly, the party is currently small enough as it is, and I am only throwing single monsters, or single elites at them. With this being the case, I have more than enough time as a DM to run a PC behind the screen. I am not bogged down by running things in this way so it seems to make sense not to over-burden the players with yet another PC to run. Secondly, I am running the PC impartially, I am not running them in a way that would compromise the DMs role as adjudicator and/or rules arbiter. I see it as a challenge to stay neutral in the encounter, and not fudge anything whatsoever.
Thirdly, the PCs need me to do it this way, they are already taxed enough as it is, and while it may be argued that I am giving them somewhat hollow victories. I have to argue the point that I am being truely neutral and playing a character in the fashion that a player with my experience level would play the same character.
I am not using companion characters, although I think it is an interesting idea. Perhaps when things progress into the Epic Tier then it will become more appropriate to do things that way, however I anticipate that the campaign will become more active with other players by that time.
The other interesting thing to note, is that I am not messing with ENCOUNTER LEVELS, I am finding variables in THREAT LEVEL of individual monsters based on this specific set of circumstances in a somewhat controlled experiment.
I have to say, that in my example the party is able to reach up to eight full levels above the party's level when encountering a single monster quite easily, and it is easy for me to stick single monsters into the storyline of the campaign. So it is kind of a win for me, makes the encounters really short though...
Without giving too much away, there is a time when I am going to expect the PCs to fight a solo monster that is three levels above the party's level and the henchman idea will probably prove too much at that point. I may try it just to find out, I have DMed monsters at the end of the Epic Tier level, so I do have a feel for how they are run. The mechanics of combat don't stifle me the way they used to after running a few 28th level scenarios...
For that, I pitted similarly leveled solo monsters against one another. Specifically I ran Grazz't against Yeenoghu and Grazz't against Dispater. I won't disclose the results of the encounters, but I will say that it is an excellent way to prime yourself beforehand to see what a monster can do. I have also done this with other monsters ranging from the Epic Tier down to Paragon Tier monsters.
I won't say that this, by any means, makes me qualified to say what it is like to run (let's say) Yeenoghu against a party of Epic level adventurers. It does, however, give me an idea as to how much math and record-keeping is required to run a monster at that level. I figure if I can keep track of two monsters at that level, then when it is time to run one against a party I will have a much easier time doing so. On a side note, this practice has made DMing the lower level monsters much easier for me.
I may try to use the Epic Solo as a lower level threat with a higher encounter level, by putting it up against a party of less than five PCs, and then again I may use companion characters. I am kinda adverse to this, as the AD&D DMG warns strongly against letting players play monsters, and I kinda like to stick to the AD&D DMG wherever possible.