This card reminds me a bit of eclipse in the fact that for every creature you play, you're doing 5 extra points of damage. I suppose it also applies to your opponent, so this is way more powerful than eclipse despite costing the same. If it's stackable it's even more powerful!
Consider that fact that it takes time to draw these out in the deck, and that it's spawn condition doesn't apply to all Elements (Contrast Graveyard, which does). Lower cost creatures would be favored in the deck more - and ideal and seemingly reliable combo I had though of was:
3 Vitalities (assuming that you managed to draw this many early on) + 1 Giant Frog = 14
, 4 Cards, and 20 Damage.
1 Mitosis + 1 Jade Dragon + 1 SoR = 19
, 3 Cards, and 24 damage (assuming Jade Dragon is already on the field, if not then it become 12).
Overall it's similar to the Mitosis/SoR/Dragon combo, except that it does much better against CC, and doesn't need 1-turn windup. However it does rely more on the cards your draw early on to inflict it's damage quicker than MitoDragons - fewer Vitalities on the field make it much harder to recover from AoE CC and shields, and if you opponent manages to PC this you'll also be in trouble (especially vs. Steal).
As for whether the opponent gets the benefit from playing
creatures - I'm not sure about that, but I don't think so at the moment. (This seems to be more along the lines of Graveyard and Mindgate).