General Notes

My heroes are mid-game geared at this point, wearing at least full transcendent gear with a little supreme and ultimate splashed in.  So naturally, if you're as geared or more geared than I am then this will workout just fine.  Otherwise, I would recommened insane difficulty or doing an easier map on nightmare before attempting a push to wave 30/30 here.  I also typically have 4/4 heroes loaded up in order to soak up XP and allow for more item drops.  Reducing the number of heroes present for the waves can also help lower the difficulty a bit if you're having trouble.

The concept of the build is simple: Build a defensive stack at the north and south crystals, aiming DSTs in such a way that you have complete map coverage.  Double lightning tower stacks at each help clear out smaller enemies while your hard hitting DSTs put in the heavier hits on the tougher enemies.  I make use of my tower boost monk in order to get through the tsunami of ogres that happen in those late waves.  Important: Make sure that when placing any minion walls that it's far enough away from your crystal/defenses that an ogre can't reach them while attacking the wall. 

Note 1

At the southern crystal we want to only use 2x 1DU worth of reflection beams to protect our tower stack and crystal.  A 5DU physical beam is stretched from the corner of the wall in the south to the statue in the northern section of this area.  This is enough to prevent late wave ogres from rushing your defenses.  Minions are used to create a wall-off to the south of this crystal as well as to the north on the stairs leading down to the mid crystal.  Finally, a 3-stack of the usual auras here placed in such a way that it expands outwards towards the southern path and towards the main middle area.

Note 2

Up in the north, we do the same thing.  2x 1DU reflection beams to protect our tower stack and crystal at the same time.  Triple stack of auras placed to the northeast of the crystal so that it expands outwards towards both paths enemies will be taking to arrive at this crystal.  We then utilize minion walls to close off both entries to this area from both the east and the northwest.

Note 3

Our center crystal is not going to have any towers built at it since the DSTs and lightning towers have it covered from a distance.  We still need to protect the crystal itself though as there are ogres that path into this center crystal.  To do this, we simply place a minion wall in the path to hold them off while using a two aura stack of ensnare and strength drain.  Strength drain will help our minions tremendously in staying alive as the ogres will deal less damage to the wall they create, and the forward placement of our ensnare ensures that our minions + DSTs have as much time as possible to take care of enemies before they even reach the wall.  A 1DU reflection beam is placed nice and tight against the crystal to prevent any ranged attacks from cutting through our walls.