easiest way to beat the game(for me anyway) is : get all the Gold Mines and Gold related cards you can, a couple Iron and Crystal cards, whatever instant cards (soul, gems etc) and all the low level weak cards (Fresh Skeletons, Peasants, Mud Golems) you can hold. Then get KAMIKAZE.
Kill enough to get the souls you need then use the Kamikaze card and just send all the troops you can. every time one of your troops die you will damage the enemy's portal -2
If you have any heal cards use them because if you kill any enemy you will loose -2 but since you are using weak troops you will die more then you kill.