Phew! Just finish all 16 stage rewards with only 400 more than required 70K points before running out of sigil. Very close call!!
I lose twice, and forced to not finish all room 4 times, due to the death of main attacking troop. I end up at exactly Level 250, just the middle of the full progress!
I only buy up to Tier IV (410 gems), so you could finish all reward stages for just buying new weapon. As I didn’t have a perfect run, It should be possible to go higher, or stop farming at around Level 200.
Noted that if you decided to farm on Level 20 instead, with the same amount of sigils as me, you would get around 16,800 (24×700) points less, so you have to spend more gems if you want to finish last 2 rewards stage.
Here’s the screenshot.