This event is really straight forward, farm the maps for a chance for Maribelle to drop. She drops on Maps #2 , 4, and 5. Map #2 has a 100% drop rate for your first clear, so even noobs should be able to get a single copy.

Silver Drops this time around are:
Bagan (Bandit)
Cecily (Rogue)
Valerie (Mage) + Soma (Archer)
Calliope (Witch)
Leanne (Heavy Armor)

The important thing to note for Maribelle is that among Heavy Armors, she's a special case. She's a possessor of the rare "Holy Awakening" Skill which doubles both attack and defense at the same time, making it one of the strongest skills in the game. To counter this though, her stats are down right awful for her rarity.

For stat comparison.
At 50CC50 with 100% affection, Leanne has the following stats:
1813 HP, 209 Attack, 426 Defense

At 50CC70 with 100% affection Maribelle has:
2229 HP, 298 Attack, 341 Defense

Maribelle only has +416 HP, +89 Attack, but -85 Defense. Keep in mind that because Heavy Armors have naturally low Attack, very often the enemy will be Defense-Capped against them. So in practice, the +89 Attack will often only transfer out to +9 Damage. An because Heavy Armors can tank 3 enemies at once (4 after Awakening), the -85 Defense will be multiplied to all 3 (4) enemies at once resulting in taking 255 (340) extra damage per set of attacks.

Copy pasted info