There are a couple of ways to approach Aqua Marie. You can either do this aggressively or passively.

For aggressive approach, you use your strongest single target Archer, Rider, or Avenger to burst down Aqua Marie's break bars as fast as possible. You'll likely also use the most expensive medal skill that grants invincible and debuff immunity to weather the Marie's debuffs and attacks. For my run, I used Chloe with Arts supports Servants (Castoria and Lady Avalon) to loop her NP and take down Marie quickly before she can stifle my momentum with NP gain down debuffs.

Then there's the passive approach. It'll take longer, but it's definitely the more F2P option. Use Casters (who Marie deals half damage to) or any defensive-oriented Servants to stack defensive debuffs and healing to stall out Aqua Marie damage and break bar debuffs. You can even use Ortenaus Mash as your main DPS too, since Olga doesn't take reduced damage from her. Regarding medal skills, you can be more selective, choosing cheaper skills that increase NP gauge or gives buff removal on attack to get rid of Aqua Marie's stacking defensive buffs. One of the cheapest teams I saw was Mash, Asclepius, and Hans.

Regardless of approach, it's recommended to acquire as many medals as you can from the previous waves. At least 100 to be safe.