This is about speeding combat in 13th Age, but the general advice works in any rpg. In fact the math works great in most d20 games, from 3.0 to 5E.
Here's the problem. You want to increase the challenge, so you add difficulty, but now your combats go on and on and on. What happened to those 20 minute combats you loved when you first found 13th Age?
To start off, look at Building Battles, page 186 in the Core book. If you follow the advice there, most of your battles will be quick. Great! But once your players start to master their characters, you'll find them too easy. Not great.
So, you opt for a combat 1.5 times or 2 times as hard as a standard battle. That's easy, just add more monsters! And now your battles are more challenging. And long. Too long.
13 True Ways to the rescue! Check out Leveling Up a "Book" Monster and Customizing a DIY Monster on page 159 of 13 True Ways. We're going to combine these to up the challenge while making combats faster.
For each monster you add, you have to manage their position and tactics, roll to hit, track hit points, and so on. Players have yet another target to worry about.
So: Start with a balanced encounter, then up the Levels and apply the Offensive customization.
For example, let's add one level to each monster, and max out the offensive customization. I like the offensive customization to speed up play, as the players miss less, making combat faster. However, the monster misses less, increasing the challenge for the players.
- Attack Bonuses: +1 for level, +3 for offensive = +4! Yay monsters!
- Defenses: +1 for level, -3 for offensive = -2. Yay PCs!
- Hit Points: * 1.25. A bit of a buffer for all those extra hits the PCs will be doing.
- Damage: * 1.25. More hitting AND more damage.
- Specials: * 1.25. I apply this to initiative, since the PCs will hit more often, you don't want the monsters to go down without getting their licks in! (Exception: I don't do this for slow monsters, as being slow is part of their deal. So I might just skip this for the slowest monsters in the combat.)
So let's apply this to a medium red dragon (is that a red dragon who tells fortunes?). They're on p. 220 of the Core book.
Changes:
- Level: 6th +1 = 7th level wrecker
- Attack Bonuses: +11 +4 = +15 for both the Fangs, Claws, and Tail and the Fiery Breath attacks. No more missing!
- Defenses: AC: 21 -2 = 19; PD: 20 -2 = 18; MD: 16 -2 = 14. 10% more hitable.
- Hit Points: 90 * 1.25 = 113.
- Damage: Fangs, claws & tail: 8 * 1.25 = 10 each; Fiery breath: 10 * 1.25 = 13 on 2d3 PCs.
- Initiative: +11 * 1.25 = +14.
- Resist Fire: 12+ * 1.25 = 15+
How's that? The challenge is upped, because this dragon will go near the top of the initiative order, hardly ever miss, and will do more damage. But the lower defenses will mean the dragon goes down faster, ending the combat in fewer rounds.
You can do this for all, or just some of the monsters in an encounter. Leveling up by itself without the offensive customization will be faster than adding monsters.
By choosing a creature like a dragon who attacks multiple times each round, the downside of the "glass cannon" effect you can get with the offensive build is minimized. Even if the dragon only lasts a few rounds, it's going to hit a lot of PCs for damage.
You can do with with the other creatures in the encounter. Perhaps your enhanced red dragon has some enhanced kobold dragon-souls (13th Age Bestiary, p. 125) as friends...
Fast AND Furious!