In this video you will see what strategy should be used to defeat Grand Widow Faerlina and complete Hearthstone Druid challenge in less than 3 minutes. This quite a tutorial for this challange.
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My top tips:
1. Start will low mana cards.
2. Use as many cards as possible, keep as little cards in your hands as you can. To avoid
3. Try to inflict as much damage as you can as fast as you can.
The reward for this challenge is Poison Seeds: http://hearthstone.gamepedia.com/Poison_Seeds