The side-lying hip adductor is probably the best way to purely isolate the adductors. Dont be shocked if you can't lift you leg high off the ground.
Get into the side-lying position towards the bottom half of the bench so that your bottom leg can dangle off the bench. This increases the range of motion and makes the exercise more effective
The glute bridge squeeze strengthens all the muscles that maintain good pelvic and lumbar spine positioning. You will need a foam roller in between your legs to do it.
You can also do this exercise one leg at a time to increase the difficulty.