Yeah Cynthia has pretty hard hitting bulky pokemon in the early to late 70s
Have you defeated Elite 4 rematch for BW or black tower/white treehollow for BW2. I find that to be pretty good measuring benchmarks to weather your pokemon are high enough level to beat Cynthia.
There is also a level ~70ish volcarona that you can teach toxic, quiver dance, bug buzz, flamethrower with insect plate setting up at 4~6 quiver dances depending on how much will o' wisp hits and sweep. Toxic is only there there to laugh at spritomb double team. You can use areal ace as a alternative to toxic if thats your thing. Just be warned that each time you use a attacking move there is the potential to get sucker punched(also volcarona doesn't have really good physical attack/def)