My personal tactics were to buy a dual machine gun, upgrade it's damage as fast as possible(the 15 starter points help out on the challenge map), soon as I've got my first upgrade in that, put up a decoy tower and mount machine guns on the decoys fast as possible. Against the Kongregate Juggernauts be sure you're packing the Apocolypse Cannon, all of it's upgrades going into firing rate(even then you'll be holding out and praying for it to recharge). Might be easier if I had any skill with the main cannons, but I don't, so it's machine guns for me all the way.
The "no spam units" thing adds an interesting degree of strategy to the game. That said, there are ways around it. The first is that you should spawn seven units(archers with a freeze or stop ability are my suggestion, they're fast firing and the status effect will keep the target from fighting back, buying you time), let your mana fill, and then rapidly click on your favorite low cost unit. I've gotten something like fifteen units on screen doing that because of the delay before the new unit registers. Also, ressurection can be performed on fallen units, which counts towards your 8 max, but can also be done after you've reached beyond that 8.
Good game, balance is a little off. I haven't played "hard mode" without first stocking up through easy mode(which does make "hard" mode ridiculously easy). Also the English is laughably bad. I suspect some of these faults are intentional, however my main concern is that the game is slow.
 "StormWinds 1.5"){kind=icon}
 "My Little Army"){kind=icon}