Looks like an invader zombie from Hostile worlds is doing something bad to your game:
"at hostileworlds.entity.EntityInvader.func_70636_d(EntityInvader.java:173)"
I would:
a) Make a backup of the world
b) Edit the modpack to remove hostile worlds temporarily
c) Load the world without hostile worlds. This should remove all invaders and portals.
d) Re-enable hostile worlds and see if you're ok now
It will remove any active portals and all invaders, but it might get you past the crash.[DOUBLEPOST=1401057184][/DOUBLEPOST]
Interesting. Mobs always try to swim up when they're in water, so surprised they aren't getting up your waterfall! But if it works, it's a cool solution.
rather than doing this i would suggest using an NBT editor to edit the entity out of your save you can get the location of the entity from your crash report and it is a 5 min job to cut that info from the region file. this is what i use to fix ticking entities on my servers and will work just as well on a single player map after running modded servers for 18 months i have never lost more than a block/entity when a chunk became corrupted (removed the block/entity with the editor) if you need more detail on how to do this let me know
Cheers Dez