I've been meaning to post to this thread for a few days. I've done an insane amount of modifications and experiments with solos having run a level 1 - 28 campaign. I'll post up my ideas, some successful, some probably just broken (but effective against my Epic group none the less).
I'll start with the demon lord Shaktari, Queen of Mariliths. Basically I took a regular marilith stat block, suped up the level and brought her up to solo stats, and tried to anticipate the problems she'd have killing my group. Remaining fair, but challenging, while also minimizing my clerical work during combat, I came up with the following. Fair warning, she is scary, but when you've seen what a level 28 party can do between damage, locking, healing, and resurrecting, you'll forgive me.
Just some design notes:
1. I can't be bothered with Variable Resistance for demons during combat. I give them a flat Resist 10 All Elements (fire, cold, lightning, thunder, etc) or more for the big ones, and if they're someone who should be more resistant to a certain type I add that in (like Orcus and necrotic), usually as immune.
2. I halve prescribed hp for all monsters and increase their damage (usually 150%, but for dragon breath I tend to go 200% or 300% because it should be the scariest thing in the game in my mind). It helps move things along, obviously, and the way I do a lot of solos it doesn't greatly affect their impact. Which leads me to a solo design I never quite shared with my players, but they did pick up on eventually...
3. Solos have something I call 'narrative stopgaps', though a more accurate name could be 'narrative invulnerability'. Yes, I know, that sounds horrible and contrived and counter-intuitive to gaming as a whole, a DM's greatest sin, but hear me out.
Like many people, I usually run solo combats through multiple 'stages', each one changing in some way, be it an evolving terrain, added solo powers, or using different stat blocks for the solo entirely. In order to graduate to the next stage of the fight, an objective (or objectives) must be met. This can be as simple as reducing a solo to 0hp (he or she gets a boost next stage), bloodying the solo, but more often it involves hp in conjunction with the story or terrain, using rp, problem solving, creative uses of powers, lore, diplomacy, and/or skills.
So beating up the bad guy isn't wasted, but once he or she is reduced to 0, it becomes effectively 'invulnerable' because the next stage is still waiting on some other unfulfilled component. Once that is achieved, we move on. And again, this isn't every solo fight, but a good deal for my games. And they don't have to be so artificial in objectives, say pulling four levers to release the water and flood the mythal, it could be organic. I've gone into a solo combat with the objective being 'everyone is satisfied' and that has worked. It's a risk, though, and if you haven't built trust or really understand your group or the story, it's going to smack of something false.
4. Solos don't have encounter powers, rather everything on a generous recharge, usually 4-6, but I'd go 3-6 or even 2-6 if I really wanted to see the power used more than once in the combat.
5. I've played around with condition mitigation. For Shaktari, I opted for 'demon lord's succor' which simply washes off a condition at the start and end of her turn as free actions. I wouldn't always use this approach, but her accompanying marilith guards will be easy targets for conditions by comparison, and technically conditions will get her, its just not for as long as everyone wishes. Makes her seem quick, slippery (especially with teleport), and of course deadly.
6. A lot of solos right out the book couldn't touch half of my high level players. I boost their attacks a bit, but at the same time I don't want the combat to be auto hits back and forth from the outset. Shaktari has a 'Your Defenses Are Weak' which gives a cumulative +2 to attacks beginning round two and stacking every round after. I think it's enough to scare players into killing her quick cause they weren't planning on that before, hehe.
7. I was on the fence about the 12 attacks, just because of the time it would take to roll and calculate, but when you have a demon lord with six arms, it just can't be helped. The actual rolling and rolling and rolling really built up suspense and even a little despair, which helps in a demon lord fight.
8. Her minor is wicked, and with the potential for ongoing. I was also on the fence about this, cause I could have just tacked on ongoing instead of wasting time and energy with a secondary attack, but you have to balance efficiency with effect. The secondary attack drew players in, and it gave a few PCs chances to help in new ways via interrupts and immediate saves.
9. Her triggered action is even MORE wicked. Whenever someone misses her, she attacks them outright, for free, for as many people miss her on a turn.
10. Believe it or not, I was considering having her act more than once a round. I'm still thinking I should have, maybe at a cost of reducing her multiple attacks in half for that second turn. Hm, you know, that would be cool. She could even do it again to reduce those attacks in half again.
Something like:
Free Actions
I Will See You Dead * At-Will
Effect (immediate interrupt): Shaktari may take another turn following an ally's turn. Her number of attacks are halved using Rain of Unrelenting Steel and Shaktari's Dance of Death. For every additional turn, her number of attacks are halved again, (ex. 12, 6, 3, 1)