After Anarook's words of wisdom I do agree that in its current form its fairly overpowered. Parallel Universe does give you added creature advantage (1) for 7
however one is allowed to select which creature to duplicate, which in itself should be worth at least 1
out of the 7. This allows for strategic choosing of extra abilities per turn.
At the current cost of Arc Spark, it -is- an instakill, but it still takes into consideration HP, as zanzarino prefers. In this situation however, the greater the total HP of all the creatures, the stronger the skill due to the splash effect, which can be as weak as Thunderstorm but as strong or stronger than Rain of Fire. The splash effect is inconsistent and the instakill does not permit the player to select a target, unless it has 1HP.
It is because of the uncertainty and average power of Arc Spark that I would say 7-8
should be a good cost estimate of cost for Arc Spark.
EDIT: Oh, and if possible, leave the name of Arc Lightning for both versions. I think it sounds better and both cards do the same things, but one is for cheaper.