The trouble with most ambushes is that with every person you are hiding you have another chance to be spotted. The best way around this would be to have one villain in the group who is very good at hiding and spotting cover the approach while the rest of the ambushers take positions over a ridge or behind an outcropping or in some other place that offers total concealment, so, assuming the targets approaches in the direction (and mode) the ambushers are expecting, the targets only have a chance to see that one guy, who if he has the time could probably take 20 anyway.
The ambushers would need to find a particular spot for such an ambush, one where they can come into striking position with just a move action, and also where using that surprise round to move provides them with other advantages (flanking, higher ground, cover) that offset the idea that they might not get the first shot, depending on initiative rolls. Still, there's advantages to be had from this and anyone risking their lives to plan an ambush would certainly put the work in to choose a spot as I've described.
--Z