^Yeah it is crazy and hard to evolve Sliggoo but you get a final Dragon Pokemon evolution from an already evolved Dragon BEFORE level 48 which is pretty rare. Kingdra is pretty much the only second evolution Pokemon to do this.
edit- I was wrong. I forgot about Vibrava evolving at level 45. As to getting Goodra, it has to be raining AND level 50. I just looked it up. I thought Sliggoo would evolve at level 40 and up in rain.