I forget for the moment, and the books are not near me at the second. Can ranged attacks - say by arrows / bolts - deal subdual damage (with the typical -4 to attack, of course)? I know that slings can be used in such a manner - but they deal bludgeoning damage, not piercing damage. Is there a rule against ranged piercing attacks able to deal subdual with the -4 penalty to attack?
If not, a group of archers with high BAB might be able to ambush them. If they attack from all directions and from elevations (such as high tree branches in a forest along a path, or from nearby hill tops in a rough landscape) then it will be harder to deal with them through ranged area effects, and if they outnumber the party then even if the party splits up to deal with them some will remain unthreatened - and perhaps ever further away as the party rushes to deal with the others.
Arm them with short swords and bucklers, have perhaps double the party's number, and aim for those that appear to be casters first. If you aim for the barbarians / fighters / etc last then there is a better chance of reducing the number of attacks (due to reduced number of party members). Then just concentrate fire from the several remaining archers to take down the brawn. You might even throw some real damage in there - to lower their threshold for non-leathal damage (not to mention there is no penalty to attacks, making stikes more likely to hit).
Unless you are using ranged attacks - and unless you take out the primary (and even secondary) magic users first - then your chances for success are rather low. Magic simply grants too much of an advantage. In an arena fight, which would you think more likely to win: a 8th level fighter with full allotment of level appropriate magic gear (armor and weaponry mostly) or a 12th level fighter with first level gear (ie: most likely masterwork weaponry and armor)? Surprise attacks and ranged attacks to weaken them prior to direct melee combat is all but necessary if magic is not an option - espeicially if the party is of higher level than the thugs or mercenaries hired to subdue and capture them.
Even with ranged attacks, surprise attacks, and sneak attacks on your side, even with aiming for the magic users and lower hp targets first, even with dealing some leathal damage to bulkier hp targets to better weaken them without penalty to attack, expect some casualties. If you have double the party's number, if you are at least the same level as the party - in fighter / rogue / barbarian / or even ranger levels rather than warrior levels, and if you have advantage of high terrain, surprise, etc - I would still expect half casualties, simply due to taking two major handicaps - penalty to attacks due to dealing nonleathal damage and lack of magic use.