I just got the hard achievement easily... If you stuck in the collecting skulls missions, here is a trick. Equip the "Subzero Antennae" and other high energy equipments. When the mission start and monster show up yet, press the "space bar" and hold on it, so that
you will go back to the out screen and army can't find you, but the monster still can use skill. Then use "Chilling Ray" to freeze people on the ground. When you run out of energy, just show up and reap it...
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