Easiest place to get water trident?

I am trying to get water trident, and looking for the easiest 100% assured mission i should do. People have told me that bosses in both Ice Caves and Thunderhead keep have it, but i dont want to run through and find out that they arent there 100% of the time.

Does anyone know a boss on a mission/out door area that you are sure has a 100% chance to always be there and has water trident?

Thanks

Or, if you dont know one that has 100% chance, a mission that has a boss relatively easy to get to that is sometimes there?