They can be, depending on how you use them. One trick that works pretty well is to camp a big spawn on your dps hero and use all DU to guard the other spawns. You can put a few dps towers/auras on your spawn area and let the app guardian + your dps demolish it. It should be considerably easier this way. Also, if you have a few decently gear friends (each preferably with an app guardian), each person can camp one choke point and you'll find that it's easy to get to wave ~15 on survival. At some point, the waves don't really get much harder, just more tedious
Also, just as a reminder, the maps are WAY easier with at least one extra person. Managing repairs/ogre spawns is almost impossible solo, at least until you've got substantial tower gear
This post was edited by Gr4v1ty on Dec 29 2011 10:38am