Balls passed multiple times on the enemies without killing them. But I retried it, and this time, 2 or 3 hits and they die. This time, I made a new game, and there was massive lag on level 7, with of course a loooooooot of enemies and 8 or 9 balls I think. But hey, this is a fun game, really !
FYI: I go in the middle of a side, I wait until every line comes to me, I start going down, then I go on the other side with a boost and get all the bonuses, then I start again. Pretty solid strategy :D