I would have to disagree. An only 4 EV SpAtk Gastrodon's Ice Beam does about half of Roserade's health.
This is assuming neither health nor SpDef EV investment. I know Roserades don't usually run this anymore because technician makes the offensive version more attractive, but Roserade used to be commonly used as a special wall with natural cure.
4 SpA Gastrodon Ice Beam vs. 252 HP / 252 SpD Roserade: 94-112 (29 - 34.5%) -- 5.2% chance to 3HKO
(94, 96, 96, 98, 98, 100, 100, 102, 104, 104, 106, 106, 108, 108, 110, 112)
is mr mime usually a good support choice?
I was thinking like phychic baton pass reflect light screen. or should I switch out phychic with encore?
Mr. Mime is the only pokemon that has an ability to block perish song and also learn baton pass (at least this was true last gen), so if you have a dedicated baton pass team you should use him. Otherwise probably not.