I am 18th level Alchemist and I am spinning my wheels against our enemies now. I drop great magic on them and they beat the fort save. Any ideas?
Gonna need some more information. What are your stats? Your feats? Your discoveries? What enemies do you usually fight? What do you usually do in a fight?
Click on my avatar for details. Our enemies are mythic homebrews with high fort saves. I am looking for something disabling with no saves or a way to get more than 3 attacks per round.
Honestly your character looks plenty solid.
If you're having difficulty doing damage then you're badly missing the Force bomb discovery, force goes around resistances so it's perfect for those high level outsiders whose resistances are all over the place.
The wings discovery saves you a lot of actions getting your flight out. Which is really a must have for high level play.
Don't bother with effects that proc fort saves. The only enemy type likely to fail the save is "humanoid with class levels" and then only the classes with low fort saves.
Is the great magic you're talking about from your Philosopher's Stone? If so, the DC's on it are quite low. At DC 20, most monsters out of the books of the right level only fail on a 1 and at best only fail 20% of the time. It's not that the monsters are homebrew, it's that DC 20 is pretty much a cakewalk at that level.