Quoted By:
Mono-typing. Very few Pokemon can get away with being mono-typed unless they have exemplary traits, and Mega Blastoise doesn't really have any. To put it in perspective, there's only seven mono-typed pokemon in OU.
Bulky, but doesn't have recovery in any shape or form, and it doesn't have the raw power to make up for it.
Rapid Spin, but you're better off using Starmie or Excadrill, or just a defogger, and using some other, better mega.
