One of the zombies is scorched by Chloe's summoned flames, but not enough to stop it from moving. Sabian moves up near the flames and stabs at the zombie with his spear. Vidrik chops at the skeleton near Cassandra with his sword, finishing it off.

On the west side:
The zombies push forward. One is shoved out of the flames, only for two more to pass through it as they try to crowd forwards and devour the living.
Spoiler
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The only way to maximize the number of zombies attacking is for several of them to get burned.
Bonfire damage to
ZB (1d8)[4] Dex save (1d20-2)[5]
ZC (1d8)[2] Dex save (1d20-2)[6]
ZD (1d8)[5] Dex save (1d20-2)[6]

Vs Mustafar
ZA (1d20+3)[11] for (1d6+1)[4]

Vs Sabian
ZB (1d20+3)[10] for (1d6+1)[5]
ZC (1d20+3)[5] for (1d6+1)[5]
ZD (1d20+3)[8] for (1d6+1)[6]


On eastern side, the zombies pursue but can't quite reach Cassandra and Vidrik.

Spoiler: Map
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=> Mustafar, Cassandra, Chloe, Sabian, Vidrik