Well, I just looked into Mass Destruction skill and wiki says that the damage is purely based on HP. IMO, I would get it if you have high HP but I'm not saying you should go pure HP. As long as you can get around 40k HP I guess that's fine.
And for the Card Summon, it's a pretty good skill. I don't know if people use it for GvG but for me it does roughly 70k crit on mobs. And I think that's the most powerful Meta skill.