you dont need the 1st 70/6 map to unlock the 2nd 70/6 map; the 2nd 70/6 map is unlocked by clearing the 100/2 map. has been that way for quite some time and i dont think it has changed.

@yoxal you can try i guess. I'm not too sure 2 silver bishops is worth leveling and I'd use anelia myself for burst damage to take out the armors when they clog up too much in the middle, but then again I havent been going for the 500 kill strategies, aiming just enough to get 400+ and pretty much managed 450+ on my accounts. the last wave before decius comes is at 465 mark, so even accounting for some leaks managing 450 isn't all too hard with some planning and enough magic damage, while 500 kill most likely require different stability in both dps and healing.

on another topic, you should max out nezha's skill with rainbows while she's AW1 if you haven't, in preparation for SAW.