So I suppose we are talking 1 high level caster, not a bunch of lower level ones ganging up on a party. If that's the case, you should get a stealthy character (or two) with all sorts of buffs to silence him, make him invisible, etc. so that he can pounce on the high-level caster when she is unaware. The sneaky guy(s) would only do one thing - grapple. A high level caster may have a contingency to get out of it in a flash, and if so the next task is to find out where he will go and be ready for him - with another grappler.
Once grappled, a high level caster may have Still Spell or other such feats, in which once pinned the grappler(s) should try to do some damage to make force a concentration check. Hopefully more help will be close by to the caster and can back up the grappler quickly by casting Silence 15' on the grappler, not the caster.
Which class is most adept at the above? It would depend on what components you may place more weight on. Getting to the caster may be the toughest part, so a rogue would be best. Or if the caster is a good grappler or can run faster than anyone, use a Str-based Monk. I would use the two of them in conjunction for flanking and if the caster somehow shrugs off one of the grapplers. If the caster has Detect Invisible or other enhancements, the rogue would probably be the best to make it to her first followed by the fast moving monk. Enter the rest of the party to help with 'subdual'; the party casters to start casting Save spells, like feeblemind if they are high enough level to make the DC at least a 25% chance of missing for the caster, and the party tanks to wreak some serious damage on the caster if she tries to cast anything or disarm her if she tries to use an item.