Hearthpwn

This deck is pretty straight forward. Try to get to the point where you constantly have more than two minions on the field so that her Hero Power won't leave you unable to do anything during your turn. Taking over her minions is an effective way to achieve this. Use Silence Spells and Battlecries to avoid her having too many minions on the field. Using Shadow Madness on her minions is an effective way to get rid of her minions; the most effective would be for example taking over Haunted Creeper, since this minion's Death Rattle will leave you with two extra minions on the field, thus reducing the effectiveness of her Hero Power. If these minions get returned to your hand, they only cost one Mana each, so playing them out on the same turn as other minions will increase the chances of the other minions staying in play after she uses her Hero Power, and preferably until your next turn.

Remember that Maexxna will ALWAYS attempt to use her Hero Power before she does anything else. This means that playing one or two minions with a Battlecry, when not having any other minions on the field, will not have you risking to lose them, as they will be returned to your hand at the start of the next turn.

When you have the board advantage, you should try to always play out at least two minions each turn, prioritizing the minions with the most effective Battlecries, as minions played with no immediate effect will risk getting returned to your hand without doing anything (apart from reducing the chance of other minions being hit by her Hero Power).

If you play this correctly (and with some luck), you will have a full board of minions every turn, and she will eventually end up having just one card to play each turn. Making valueable trades with her minions and then dealing as much damage as possible on her each turn will most likely let you whittle her down over time.

One important aspect of this deck is to prioritize minions' Battlecries according to which part of the battle you're in. For example, Mind Control Teck is a card that is effective when she has four or more minions; playing it in another situation will most likely result in it being returned to your hand without having any effectiveness.

Try to balance aggressiveness and control in trades and minions played, as too much agressiveness may lead to ineffective use of minions with Battlecries. On the other hand, playing too few minions will result in you not having any presence on the board (especially after the use of her Hero Power), thus leaving you vulnerable to losing board control.

If you have questions/suggestions, feel free to leave a comment, so I can get back to you as quickly as possible!