Just to clarify, Haka's GP states that you play the card. Once played, you either have to discard two cards, or destroy GP. Now, correct me if I'm wrong, but if GP is destroyed, the damage-negating effect of the card goes with it, right? That said, if you're just going to destroy it, what would be the point in playing it in the first place?
You got it--once the card is gone, so is the effect. Think of the two discards as your "cost" for playing Ground Pound. If you don't pay the cost, you don't get the awesome effect.
A card's effect will only stay in effect after the card is gone if the effect has a specific duration which hasn't been met yet. For example, I think the power on Ra's Flesh of the Sun God makes all heroes immune to fire until the start of Ra's next turn. If Ra used that power on his turn but was then incapacitated, everyone would still be immune to fire until Ra's next turn came around. The same goes for effects caused by a target (such as Akash'Bhuta's Ensnaring Brambles) if the target is destroyed between turns.
A reason for Ground Pound to be worded the way it does is Omnitron-X's Timeshift power, which puts the top card of a deck directly into play; if you TS a Groundpound, you've wasted your power unless Haka has the cards to discard. (Combine this with his Haka cards, Punish the Weak, and the multiple copies of Mere and Taiaha, and Haka is generally not a good target for Timeshift.) Similar things happen if you fight in Atlantis, where the Font of Power plays the top card of everyone's deck, or against the Ennead if you kill Set early (not generally recommended, by the way). All of these can cause Haka to play Ground Pound even if he's not quite ready to.
Killing Set first is my FAVORITE way of beating the Ennead. Just throwing that out there.
Killing Set early is usually my biggest mistake when playing on advanced. At least it always feels that way. They always happen to crush me the exact turn after Set dies every. single. time.
I've been able to work killing set as an advantage most of the time, including the time visionnary used mental divergence to keep set playing ground pound every turn.
My first game against the Ennies, I was like "Extra villain plays? SET MUST DIE! ...Aw crap."
Of course if you can Mistbind or Take Down the Ennead first, killing Set is likely to be tons of fun. But both NM and Legs have a fair number of cards you might not want to get as free-plays.
My thinking is that an Alive Set only has a chance to churn out extra villain card plays. And they all suck on advanced. Dead Set is 100% certain to churn out extra villain cards each turn.
Is he really more of a threat alive than dead given the context of Advanced mode?
The cards that work best with a downed set in my opinion are in order: Reclaim from the Deep, Vernal Sonata and Don't dismiss anything. If tempest is the last hero and he can spend the rest of the game playing only reclaim from the deep and still set up the best card for all other heroes. Is it a boring repetitive play kinda, but hey you'll still get a surprise each turn when set plays your card.
The best way for me to beat the Ennead is for Set to remain under the Shrine for the duration of the match!
I always have that moment as Tempest where I forget into the stratosphere only targets villain cards, and start thinking of cards that could be put on top before DDA come out, then I look to see if it says ongoing card or just card, and see that villain word staring at me, ruining all my schemes.
If Fixed Point (environment card from the Time Cataclysm deck) is in play, you can play out Ground Pound for free. If its wording were simply "Discard 2 cards to play this card.", you wouldn't be able to do that.