After many, many attempts, I finally got it. Here's how:
After just a little bit of leveling, the only serious challenge is the ninjas. Nothing else, not even non-ninja bosses, can come anywhere near the treasure unless I get very careless. So there are just 2 things to deal with...
1) The little ninjas die in one hit to a meteor, especially if you upgrade damage at least once. I prefer to wait until they're about half a second from the treasure before using meteor, to give my towers a chance to kill them (which saves mana while giving exp). But be sure to draw a mental line on how far you'll let them go before casting. Maxing out max mana, mana regeneration and minimizing meteor cost will help keep you from running out of casts.
2) The most frustrating enemy in the game, by far, is the Wave 40 ninja boss on the last stage. Even if you hit him as soon as he appears on the screen, he's so fast that he gets to the treasure well before his invisibility runs out. In fact, his speed is so broken that he'll be at your outer defenses when he finally loses invisibility. So you'll usually only get to hit him with one bullet from a tower. I finally beat him by building level 7 banshee towers (upgraded versions of ghost towers) at each entrance and trying again and again until the first bullet to hit him was a 'disarm' (removing invisibility). He then died quite quickly, with a little help from meteor casting just to be safe. Obviously, max out meteor damage and undead exp gain.
Also, as a general tip, try to upgrade your towers in order from those closest to the treasure to those farthest away. This prevents outer towers from hogging all the exp and lets you blast non-boss ninjas with your best towers when their invisibility runs out.
Any other suggestions? I also tried building level 7 orc critical hitting buildings at each entrance (forgot what they're called). But that didn't work because even high ground + crit still leaves the boss with quite a bit of health; and my fingers aren't fast enough to chase him with the mouse while spamming meteor. Fear and freeze don't seem to do the trick either; both leave him with enough time to activate invisibility and run to the treasures.
If you have a hard time spamming meteors, then turn the speed down. I'm pretty sure you can also hold Shift and click to cast multiple times. I always hated the ninja champions for this exact reason, but they're not THAT bad if you have meteors available.
Oh, and one other thing, if they get caught in the splash from a Burning/Hellfire tower that's aiming at something else, they take damage even when invisible.
I did it with speedhack (need to slow game at 39-40 waves), but possible with just hotkeys. My skils: Full Orcish path, Undead mana+frenzy skills; Demons mana+exp+meteor skills. 3 Orc towers on each side enough to kill everything but ninjas. 1- 2 meteors kills ninjas, and mana fully regenerate between waves. At 38 wave i have ~1500-2000 free gold.(Chopping trees; 15% more gold). After 39 wave just build (+20 mana -100gold) and sell demon tower( +50 gold); 5 times=1 meteor. i have enough gold to 2k/50=40 times; enough to fill mana fully. Boss died after 5 meteors. Hotkeys: 3 for demon tower; E for meteor.
Archlord, how do I turn the speed down? (Apart from making sure the game is not set to fast forward)
A new challenge to keep myself entertained, maybe even keep me from quitting until I get the "max all skills" achievement.... How well can you do on the final mission without upgrading a single tower? I've managed to beat it with while losing only gem, keeping fast forward on the entire time, and without letting the final boss even touch a treasure.
