Using tf_zombie_spawners can be taking on a map with plenty of timers also in effect. Regarding them to activate and kill their full hiearchy would ease the load, but having any script think functions could be draining, as well as the incredibly erratic skeleton beheaviour, it is easy to underestimate their damage output, which cannot be tweaked. Plus, looking for a sensor to detect if only one is in the brush could exist, but I have doubts off the top of my head. So, it's possible, but could be strenuous on server load, and can be very inconsistent, and lead to accidental deaths, unless done well, which may draw into a larger chunk of map entity resources.