For little forest just build warriors on each corner from the back forward and upgrade them thus: energy>consecrate>special>tactician>damage.
the warrior energy and paving upgrades help alot as well as the energy hero that adds damage to energy units.
@Typhoon00 for mountain of fire just spam mages w/o upgrading everywhere until wave 10, make sure that on 10 you have the 6 middle squares as cold units. Then at wave 18 you can start upgrading all of them to cold units.
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