hrrmmmm...... I was on at least lvl 450 when the game glitched out and gave me a blank screen with a ! in the middle...thats fine..game had taken way too long by then anyway and I wasn't going to die anytime soon, but I didn't get either of the kill badges either. (Extreme mode unlocks after beating 300 lvls in one round btw)
Well, I've just reset the game and beat wave 1-4 without any problems on Chrome, FF and Edge.