In a custom 5e campaign I've designed, the big boss is a very powerful entity. It is not stylistically reasonable for the players to fight it below level 17. They are currently level 6. My planned campaign content should take roughly twelve (12) 4-hour sessions to complete. This means, in order to reach the desired character level, the PCs must gain one level per session.
As for why the final boss fight should be a very high-level encounter,
the Big Baddie is an ancient spellcaster of immense power. Changing this is difficult, as in our already-played sessions I've baited the players with high-level abilities, such as demiplane creation and travel.
Leveling up once per session seems unreasonably fast. I worry the players won't have time to adjust to new abilities and spells, but also that it will be difficult to convey that reaching such a high level was an achievement. In addition, the experience required to do this is very high; employing the milestone method seems to cheapen the difficulty of reaching such a pinnacle of power, but actually earning the required experience will require every encounter be of Deadly level or higher.
Possible solutions I've come up with:
- Add more content. I can certainly develop more content, adding more sessions to the campaign. This would seem an obvious solution, except we typically play 1-2 times a month. 12 sessions will take at least 6 months, more likely 9 months. I am loathe to extend this by 8 sessions (to a total of 20), which would be the rough minimum to make it not feel like a level every session.
- Change the big boss. I can make the boss different, scaling difficulty down and enabling the final encounter to be less than a level 17 fight. This would be fairly simple mechanically, but require massive changes to the plot- some of which would be difficult to pull off, since we are already three sessions into the campaign.
- Increase the leveling speed to match, as discussed above.
- Find a story reason to give more experience than granted in sessions. "You spend a week fighting bandits and get 12000 experience" would be a (terrible) example of this principle. If done well, with random event tables the players roll against and fun "scripted" events, this could allow the fast leveling required without constant near-death fighting in every single session, but still suffers from possible problems resulting from such rapid advancement.
- Something else, perhaps?
How do I best get the PCs from level 6 to 17+?
Following advice from SevenSidedDie, I'm posting the real question. We'll see if it works or if it needs to be adjusted/edited/asked differently.