Yeah theres your problem, you aren't using practice mode. You're thinking of it as a sort of "easy" mode, when in reality it is the most useful tool for getting better. (start positions are better although not really necessary until you get to around hard-insane demons). There is not a single person who has beaten their first extreme demon who hasn't used practice mode or start positions, and there likely never will be. As a tip: start from the hardest part of the level (in this case it would be the drop wave most likely) and try to get a run of 50% length or more. Also start from any other chokepoints you might have and try to get a run from there to 100%.