For future reference, here's the complete strategy I used to beat this.
Wizard level 49: I beat all of the other levels on basic settings; I did some grinding, like beating the first 3-4 levels with all the difficulty multipliers (more than 6x experience) and beating some easier levels with a few additional settings.
I also benefited from a lot of journey amulets; you can beat any level to keep adding to the counts for these.
My skill distribution was:
- Focus: 30
- Replenish: 30
- Forge, construction, multiple damage, slowing, dual gem: max
- Resonance: 11
- Recharge: 3
- Radiance: 3
My gem strategy: I built several gems the same way: combine level 1 of slow + multiple damage, then keep fusing with the next level slowing gem. I used level three gems (the squares), with some ascending to level four by the end.
My tower strategy: I placed level three gems in all the starting towers; I focused a cluster of towers around an amplifier in the bottom-left corner of the map -- three towers touching the lake, to charge the crafting pylon, and a smattering to cover enemies. I also placed a tower directly to the left of the magic orb.
I placed towers as needed, mostly in U-shaped paths to maximize surface area covered, usually lone towers.
I also had a charged bolt shrine next to the magic orb; I used it once or twice, mostly on giant monsters.
Near the end, around the second major wave of giant monsters (11k+ HP), I realized I wouldn't make it; since I had 16k+ of the orb done, I:
- Switched all gems to target structures
- Built a couple of towers in the top-left quadrant, dupe some gems, same settings
- Kept summoning the maximum wave of monsters I can see
Although my game started lagging like crazy, I did manage to finish off the last ~3k of charging before the monsters breached my tower. I had to use mana pool a few times to sustain the huge amount of mana.