Follow Slanith's advice. If you have 2 ATK down eromon (Sierra and Nikola work, but you should have their skill upgraded a few times), 2 ATK up eromon, a good healer, and a good emerald damager, you can beat it.
I used the following:
Sierra (skill level 9 for 29% ATK down) - Get in Alchemy
Nikola (skill level 4 for 24% ATK down) - Get in Alchemy
Asahi (skill level 0 for 40% ATK up) - Event
Hinami (skill level 0 for 50% ATK up) - Event
Aina (skill level 1 for 1120 heal) - Rare on stage 6
Ryo (skill level 5 for 860 power) - Event (not really a good choice actually)
The key is to have those 2 ATK down eromon, 2 ATK up, a good healer, and a good emerald damager to get Samantha past that 20% threshold quickly. Beyond this, having 3 Ruby eromon on the team REALLY helps, because a lot of the stages have you matched up against emerald damagers. Then, follow Slanith's advice:
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It's tough, but doable. Should not take more than 1 maybe 2 attempts if you have those eromon.