So I'm making a custom arena for Cheep Skipper and instead of water I have lava (Dont ask).
Everything goes smooth untill he jumps, right when he is at the level of the bridge heading down No$GBA poops out a error, a fatal error
My guess is when CS hits the water it tells the game to load a special animation for the object. But lava dosnt have the animation.

Is there a way to get around this?

Bosses crashing is usually due to putting the boss in another level than the original.

Have you edited the original level, or created a new one?

I moved it to 1-1 area 1, this may be the reason, I was going to have him be the w6 boss, but I can change it around if I need to.
Something that could work if that is not the case could be making the lava act like water, I think I saw how to do this but I forget how.

okay, that IS the reason. The problem is not the water, it's that you changed the level slot.
If you edit the original boss level WITHOUT moving it to another slot and put lava in there, it will work.

ha. ok. time to find a no$gba nsmb save