The greatest tactics to Dungeon Defenders are:
1. Don't move around too much try to stay somewhat near the crystal
2: Don't rely on your defenses too much, having defenses is a good thing but try to have them at critical choke points in the map. If the map doesn't have very many choke points you can make them by using barriers.
3. If you have teammates use team work. I know that's a bit vague but what I mean is don't get too far apart, rely on each other, and combine defenses. One of the best combos I've seen is the Series EV Physical beam combined with the harpoon gun.
4: don't don't get backed into a corner i know from experience you will more than likely be killed unless you have a very good weapon or a good team to back you up.
5: Stay calm, getting annoyed at the game and your team isn't going to help anybody except the A.I.
Those are a few good tactics to Dugeon Defenders. Good luck.
If you have anymore questions hit me up, my Steam I.D. is: Soldierboy624