Archvale

Mongo can be found in the Autumn/Fall biome (Amberpath). He's a fairly random find, so just keep searching the biome until he appears.

Fairly easy fight if you keep your range from him.

The boss Mongo is pretty easy. After i discovered this method, i did he rest of the fight without getting hit.

I recently got the spiked club melee weapon and went into the upper left of the room. I basically hid behind a tent and I was still able to hit the boss, but he was unable to hit me. Just keep him on the other side of that tent and his projectiles will be unable to hit you and you are free to keep hitting him until he dies.