After reading a series of articles about re-structuring the way solo monsters are handled in D&D 4E at The Angry DM (The D&D Boss Fight Part 1, Part 2, Part 3, and Part 4), I began to think about how this concept might be applied to 13th Age. In short, the Boss Fight system consists of splitting a solo monster stat block into 3 different stat blocks with 1/3 of the HP of an appropriate solo. This splits the fight into 3 stages which keeps the fight dynamic and provides a better sense of progress. Later-design (post MM3) sensibilities are also followed, such as increasing the action economy, spreading out solo actions, and giving them resistances to certain control conditions.
In 13th Age solo monsters don't really exist as their own discrete category. Rather, a Large monster is designed to be as powerful as two same-level normal monsters, and a Huge monster is the equivalent of three same-level monsters. In practice, I've found that they don't function that well if run as true solos, though I have heard of people using monsters several levels higher than the party as solos and having it work.
Another key to a good solo battle is a dynamic environment, which is covered in the D&D Boss Fight articles but also addressed directly within the monster design with the Worldbreaker system on the blog At-Will. Consider the difference between a generic fight with a red dragon in its treasure hoard where it simply trades blows with the party (given that a major complaint about 4E solos is that they were just "sacks of hit points" leads me to believe that this is a common scenario) and the action-packed final sequence from The Hobbit: The Desolation of Smaug (which, while arguably over-the-top, would make a pretty awesome RPG boss battle).
To kick things off I'll design a solo version of a Large Black Dragon, which is a Double-Strength Large (yep, I'm stacking them) 6th Level wrecker in the core rulebook. I've taken elements from both Boss Fight and Worldbreaker, though I've opted to keep things simple and avoid splitting the dragon up into 3 separate stat blocks when just 1 will do. Mechanically, the Solo Speed trait does a lot of the heavy lifting in terms of making this Large creature function as two enemies without changing around the dragon's attacks too much, as it now acts on two different initiative slots. While debilitating control effects are less of an issue in 13th Age compared with 4E, Draconic Toughness is still a good way of preventing the PCs from wrecking its action economy and turning it into a big sack of hit points to be beaten down with impunity.
Some notable changes from the published stat block are the replacement of ongoing damage from the dragon's main attack to a more narrative secondary effect that lets the dragon use its environment to its advantage. The effect of having 3 stat blocks is replicated by the addition of two special trigger powers that separate the 3 stages of the battle. While somewhat clunky, negating overflow damage on the dragon and the PC respites (both from Boss Fight) are written directly into these powers; this is probably less clunky than 3 stat blocks, though. The first trigger power also increases the frequency of the secondary trigger on the dragon's main attacks (in later stages it's no longer toying around with the PCs, but trying to use the environment to wreck them), and the second trigger power swaps out the Draconic Grace trait with Acidify Environment. By stage 3, the dragon is injured and desperate, bleeding and spitting acid just about everywhere. It should probably be making judicious efforts to escape unless the PCs are hanging on by a thread.
As far as balance is concerned, the fact that this dragon is getting two free action trigger powers might make it seem more powerful than 2 regular large black dragons, but the PC respites probably balance this out, plus there's the fact that 2 dragons would have two chances of using Draconic Grace each round. The solo is probably a little tougher than 2 core rulebook dragons, but not by as much as it would first appear. The more hand-wavey, narrative trigger of the dragon's main attack also gives the GM more leeway to take it easy if the party is having a tough time, or really showcase the danger of the environment if they're doing too well.
Large Solo Black Dragon
Double Strength Large 6th Level Wrecker (Solo)
This dragon has all of the traits that a Large Black Dragon from pg. 220 of the core book has, including Escalator, Flight, Draconic Grace, Intermittent Breath, Water Breathing, and Resist Acid 16+. It also has the following Traits:
Solo Superiority: The Dragon has two initiative slots, one at its rolled initiative result and the other at its rolled initiative +10. It can make saves at the end of both turns, but it only takes ongoing damage on its first turn (Initiative +10).
Draconic Toughness: All Saves that this dragon makes gain a +5 bonus. The Dragon immediately rolls a Hard Save (Normal with the bonus) if subjected to the Stunned or Confused condition. On a success, it's Weakened instead.
Claws, bite, or tail sweep +11 vs AC (2 attacks) - 18 damage
Natural 16+, or Natural Even Hit if ≤ 214 HP: the target is flung into a dangerous or inconvenient environmental hazard. Choose an option from the Sample Environments section, or improvise your own to match your game's narrative.
Natural even miss: half damage if the target was a different enemy than the last one the dragon attacked, including if the attack made on the dragon's previous turn.
Special: the dragon gains a +4 bonus to AC vs opportunity attacks if it moves after making this attack, including if it uses its first action on its next turn to move.
C: Acid breath +11 vs PD (1d3 nearby enemies) - 20 acid damage, and 10 ongoing acid damage.
Miss: 10 acid damage.
[Special Trigger] Murky water flyby - When the dragon is reduced to 214 HP it disappears (remove it from play), ignoring any damage from the triggering attack that takes it below 214 HP. Each PC can spend a Recovery and either regain a 1/battle power OR immediately roll to recharge a power. After 1 round (during which the Escalation Die does not increase), all PCs make a DC 25 Perception check. If they succeed, they can take an opportunity attack against the dragon. If they fail, they take 4d6 damage. Place the dragon anywhere nearby, and give it an additional use of Acid Breath. Note that the trigger of its claws, bite, or tail sweep attack also changes at this point.
[Special Trigger] Wrath of the injured - When the dragon is reduced to 107 HP it immediately uses Acid breath as a free action, even if it normally would not have any more uses of it, and ignores any damage from the triggering attack that would take it lower than 107. It also counts as Staggered, loses the Draconic Grace trait, and instead gains the Acidify Environment trait. Each PC can spend a Recovery after the attack from an increase in morale at seeing the dragon visibly injured.
Acidify Environment: Any enemy that ends its turn adjacent to the dragon automatically takes 10 acid damage. After one round of possessing this trait, any nearby unengaged enemies also automatically take 5 acid damage.
PD 20 HP 320 (214, 107)
Deep Water with Mucky Bottom - The PC makes a DC 20 check to avoid becoming stuck in the muck and to swim to the edge of the deep water in 2 move actions. If the check beats a DC 25 instead, the PC does this in a single move action. If they fail, they're stuck underwater until the next turn.
Thrown against a Wall - The PC takes an additional 2d12 damage and is Prone. Standing up can be done as part of a move action, but if you wants to move anywhere else you must succeed at a Normal Save (11+). While prone you are Vulnerable to melee attacks, but ranged attacks made against you take a -2 penalty.
Thrown Behind an Obstacle - The PC is on the other side of a pit, a geyser, high on a ledge, thrown off a small cliff, etc. Usually this will take a DC 25 skill check to overcome the environment in a way that makes narrative sense. Depending on the environment, a failure should either result in 10 damage or cause the PC to take two move actions instead of one to get back in the fight.