Seiging castles of nations with free summons

[Executor]

If you don't have crumble than you're pretty much screwed. They can last indefinitely, food is not a problem for Pan with all the nature gems, assassinations aren't all that useful either against Pans.
However, Pans enemy number 1 is MIND HUNT!!! Make use of it.

[SlipperyJim]

Or you can just skip the storming altogether. Camp out around his forts and preach him to death. If you can eliminate Pan's dominion, all of those maenads will go *POOF* next turn....

Tinkerbell Effect FTW!