To fractalman: In my opinion, the poison only level is the toughest so far (I have completed 28 maps so there may be harder ones to come.) I would think it must be very impressive to beat that level without spending any skill points.
To wabash500, this is the order of which I always put the available gems per map starting from spawn point to end point: steal mana, reduce armor, splash/chain hit, triple damage. Slow/shock can go any part of the map. I find it to be effective as you gain a lot of mana throughout the game to help support your other costs. The triple damage is at the end to kill any that leak by any strays that got through as it has the highest single target damage potential.
 "The Last Stand: Dead Zone"){kind=icon}
 "Wartune"){kind=icon}
 "Clash of the Dragons"){kind=icon}
 "Crystal Story"){kind=icon}
 "GemCraft"){kind=icon}