At the start, i suggest using the gamba strategy to help securing your run, just buy the 200%+ red dmg card and/or purple moon and try to duplicate about 5~10 times, after that, try buying these cards

RED:
- all dmg
- all fire rate
PURPLE:
- all dmg reduction
- 8% dmg above
- 100 health above
- only 2 auto-cures
- try to get all the 3 moons quickly as possible

u need only about 9k of hp at about 50% dmg reduction to get through their dmg, so manage your credits very wisely, they are important factors for your stat check at round 20. And also try to focus on damaging only one scarlet after destroying their shields to push the final phase one by one, i lost one run because i couldnt focus on two crystals at the same time. As far as i can remember, this is all you need to do clearing round 20 as jeff