OK, so we know the Aerplanar don't do well in the crater, so if we send ANY, we send only a couple. The Zeever spiders are designed for it, so send most. Stymph crabs may be able to distract the death gravity?
Medusa should do well in the Rivers now, and rozers are our second best bet, followed by Aer. This area also probably has the least airborne death, other than the enemy, so let's put most of the Stymphs here as we're most likely to see enemies surviving the terrain.
The bog is probably best handled by Medusa, but Aerplanars could fly over the gasses, and it's possible the Zeevers could make a little shelter against the gasses, while Rozers dig underneath.
In all, I think a balanced approach to rozers is appropriate, while we divide our remaining forces as fits their strengths and weaknesses, then fill in the gaps with grubs.
The Bog: 600 Grub, 150 Grub+, 195 Rozer, 75 Rozer+ 300 Medusa, 121 Medusa+, 195 Aerplanar, 105 Aerplanar+, 26 Zeever Spider, 9 Zeever Spider+, 16 Stymphalian Crab, 7 Stymphalian Crab+
Lava Rivers: 500 Grub, 200 Grub+, 195 Rozer, 71 Rozer+ 250 Medusa, 80 Medusa+, 195 Aerplanar, 105 Aerplanar+, 35 Stymphalian Crab, 10 Stymphalian Crab+
Glowing Crater: 900 Grub, 250 Grub+, 195 Rozer, 75 Rozer+, 35 Medusa, 20 Medusa+, 45 Zeever Spiders, 14 Zeever Spider+, 20 Stymphalian Crab, 6 Stymphalian Crab+
Plan A: (1) Flabort