This challenge is much easier to complete with multiple players since the number of goblins per round does not increase with each additional player. It is still possible to solo with good towers.
Solo Hard Squire Strategy
Build a bouncer barricade on each staircase, then 2 on each side of the ramps. Keep your character in the middle and stop any goblin who tries to get the crystal. After the first wave replace the bouncer barricades on the front stairs with slice and dice blockades. then replace the two mid bouncer blockades with slice and dices and one more bouncer blockade with a slice and dice. One of your towers will have to be a bouncer blockade I recommend leaving the back left stair a bouncer blockade since it receives the least amount of traffic. After this just get the chests and upgrade your slice and dices, make sure you stay by the crystal to stop any stragglers.
This strategy can be used on insane however you will need much higher stats and a high powered pet will help make the first wave somewhat easier.
This above strategy is good, but a way more reliable strategy which can be use with every class (simply by switching right before the wave starts). Is to make Slice 'n Dice blockades at the front end of the crystal, right between the stairs and the crystal, be sure you place them as close to the crystal as possible. After the first wave you should have enough mana to place Slice 'n Dice blockades the same way but now on the backside and on the right and left side (you won't have enough defense units to place a slice on both let and right so place a bouncer on one of those sides). After you have placed these almost no goblin will penetrate your defenses, just upgrade as you feel is necessary.
Two-player Insane Strategy
This two-player strategy has been used to beat the challenge on Insane. All you really need for this is an aura Monk with average stats (level 45 can be sufficient), a tower Apprentice (all you really need are points in tower health, but offensive stats make it even easier), someone with high casting rate, and anyone who can do some decent DPS as a hero spec. We just used the same Apprentice (70) for the casting rate hero and a DPS Squire (70) to fill the other role.
Start off by setting up 3 sets of Lightning/Ensnare Auras around the crystal - one on each set of larger stairs and another to cover the 2 sets of stairs near where the Forge is in between waves (you may choose to use a different amount at slighty different locations around the crystal, which is fine. This setup gives you ~15 defense units to play with.) Next, just make a circle of magic blockades around the crystal itself.. it should require about 10 of them depending on how you space them. After this you can fill in the remaining defense units with more auras (Strength Drains or a Healing Aura in the middle would both probably work), offensive Apprentice towers (I believe I put up a Fireball and Lightning Tower on the ledge overlooking the crystal with a magic blockade on the stairs behind them and that was it), or any other towers/traps of your choosing.
Now, all you have to do for the challenge itself is let your DPS hero fight while your casting rate hero stands inside of the blockade circle and constantly repairs/upgrades. It really doesn't get very difficult until the orcs start dropping, but in between the quick repairs and your fighter cutting/shooting them down (Mana Bomb to help out is another plus to using the Apprentice here) it should really be no problem.
Solo Insane Squire Strategy
This strategy can be used to beat it on Insane solo although it's much easier with multiple players. A tower squire can build 11 bouncer blockades, entirely enclosing the crystal area. No monsters are likely to damage the crystal unless two adjacent blockades die (occasionally monsters fall directly on top of the bouncer blockades and may end up inside the circle). If the bouncers can kill a goblin in a single hit (easily done with 70 points in tower damage), none of the goblins will damage the bouncers. Once orcs begin arriving, it is necessary to either kill off off the orcs with hero damage, or spend time repairing (save upgrades for a quick emergency repair).