The side-lying hip adductor is probably the best way to purely isolate the adductors. In fact, a study was done comparing several different adductor exercises. This one was the best.
Next is the elevated version. Get into the side-lying position towards the bottom half of the bench so that your bottom leg can dangle off the bench. Since you’re elevated- the range of motion is increased which makes it more challenging.
The glute bridge squeeze strengthens all the muscles that maintain good pelvic and lumbar spine positioning. In addition, it trains hip extension which also activates the adductors.
Or you can simply do this exercise one leg at a time to increase the difficulty. Make sure to keep a foam roller or a pillow between your legs to activate the adductors more.