Magic: The Gathering rules question. regarding sorcery damage assignment.?

have multiple questions, regarding situations came across in mtg: duels of planeswalkers 2012 video game.

1. in challenge "the tangled vale", opponent has many elf creatures, of have toughness of 4 because being enhanced imperious perfect (other elf creatures +1/+1). part of solution solving challenge use sickening dreams (deals x damage each creature , each player) having deal 3 damage creatures.
in situation described above, sickening dreams dealing 3 damage creatures enough destroy 4/4 elf creatures because 3 damage assigned first imperious perfect (2/2) creature, cause other elf creatures lose +1/+1 buff.
question in order damage assigned spells deal damage multiple targets?

2. have card i'm particularly fond of: gideon's lawkeeper (white mana, tap: tap target creature). use ability prevent particularly powerful creatures form attacking me, or prevent them blocking. unfortunately came against situation not useful: creatures tap abilities. don't recall name of creature, had ability (tap: sacrifice destroy target enchantment). tried tapping prevent using ability, used anyway.
question is, when try tap creature has ability (with tap cost), creature can in response use ability (and tap itself) , because ability on top of stack resolved first, therefore rendering tapping attempt useless?

i have multiple questions, regarding situations came across in mtg: duels of planeswalkers 2012 video game. 1. in challenge "the tangled vale", opponent has many elf creatures, of have toughness of 4 because being enhanced imperious perfect (other elf...

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