So tower events, ranking is incredibly troublesome and requires some careful management of time. Anyway, from my experience, ranking between 1000-2000 usually requires that you run advanced and hit rune time once per day, using up one valkyrie crystal during rune time to maximise number of passes gained, on top of making sure you have full tower BP at the start of rune time for a total of 10 BP to use.
So to rank 0-1000, I'm guessing that you would need to run extreme following the same method of maximising number of BP in rune time? Lol, I tried extreme once during a previous tower event and every team just got destroyed within 5 turns in the same first battle (May be slightly incorrect, been too long)... Oh god, it's so ridiculously crazy, I can't even imagine what kind of team you'd need to complete extreme consistently, and the rune bosses of extreme? I'm scared to even imagine it...