Beating level 15 in <90s is just a matter of taking down the laser tank ASAP. Get max upgrades, and keep your beam on the laser tank until your shield is JUST about to run out, then block his beam. Ignore any leftover enemies, but clear them out as fast as you can before the laser tank recharges in the previous wave. Simple and easy.
Holy crap, make engineer suck less. Right now, that 8 cooldown on attacks is crippling beyond belief. You need like 6 or 8 of them to do as much as 4 of any other ranged unit.
Great game, but like DestructoMonkey said, the melee units in this game are largely useless because they can only output damage in a very very tight area, and they are dead weights in maps with wide lanes or any flying creatures. Mages forever.
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