Here is what I do to beat it on insane:
1 Tower Squire
1 Tower Monk
As many DPS huntresses as possible (it helps if the squire/monk can swap to dps huntress).
Tower Wizard (great for repairs and mage towers)
The main things you need to watch out for are the kobolds, ogres, and wyverns, and since there is a very limited amount of DU, you have to maintain a fine balance at all three crystal positions, which can be difficult.
Here is the typical setup I like to use when doing the mission on both hard and insane:
This setup uses the exact amount of initial mana available. My monk's aura are quite large in radius, bigger than the circles show, but the positions should be what they are. The only things missing from this setup are the remaining defenses used to defend the crystals. This depends entirely on how many people you have and where you will position them.
This setup puts a decent amount of defence at each crystal, but you'll want to shift things around depending on if you have 2 people (can only cover 2 places) or more (put towers at the center where most of the enemies gather).
Easy to miss defenses on that picture:
- Spinner on top of building
- Barricade by right crystal
- Bouncer on bottom invisible path near bottom crystal
The middle crystal is by far the most difficult to defend. It is pretty much attacked from all sides by both wyverns and ogres. Put your best DPS ranged character here. They will be busy for most of the fight fending off all sides.
The top crystal is the second hardest to defend due to the extremely short path wyverns have to take to get to the crystal (they flight straight out of the door). Also, that north door tend to spawn lots of ogres. Put your second best DPS character here.
If you have more friends, the next part will help a lot. The right side can pretty much handle itself unless a mini ogre pop out of the spawn point in the middle of the map. At that point, you'll want someone there to take care of them before the barricades there breaks.
Finally, having a person run around the map to repair and upgrade is a huge bonus.
In later rounds (10-11) kobolds are your biggest threat. They come in large packs and can easily take down a fully upgraded bouncer in a few explosions. When placing your traps. make sure that you place the towers back enough from the walls so the explosions do not damage them.
Mini ogres are also really annoying, having a ton of health along with their usual ogre splash ball attack. Take them out as soon as possible.
Wyverns are your biggest threat, it helps a lot if each person guarding the crystal has a strong attacking pet that can take out wyverns while you focus on clearing the lanes of enemies.
Here are the 2 links to the pictures as I cant' get the top one to size properly so its too small to see:
Update in response to comments
The problem with crystal trackers is they are great aoe, but their range is more limited than other weapons and their DPS is not as good. It would help a lot if at least one of you switched over to a more DPS oriented weapon for the mini ogres. Again, doing the challenge on hard would score you a van wolfenstein, a nice one (3-4 split 30+ upgrades, fast reload/shoot) will make quick work of the mini ogres. The best part is their range is very long, you can easily reach the barricades next to the top left crystal while standing near the bottom middle crystal to take out any mini ogres that are encroaching on that area. Generally enemies from the middle spawn will not go up that ramp towards the right crystal. They tend to come up the south ramp towards the bottom crystal instead. The only enemies that go in the other direction are the mini ogres.
As for the assassins, make sure you are using Huntress Animus pets, they will not only take care of incoming assassins but can easily handle any wyverns coming at the crystal you are defending as well, leaving you to handle the more important areas.
In summary, this is not really a challenge that you can expect to do with less than 3 people, I believe it is designed like that on purpose as the rewards you receive are quite good. One last thing you can try is completely overloading the right crystal (put at least 5 strike towers there for the mini ogres) and just use barricades on the other 2 crystals for defense.