Tin Soldiers are immune to your charge and flame so you need to get kissed by a fairy to be able to kill them.
There is a secret part of the level that has a lot of tin soldiers and a fairy that will grant you an unlimited amount of fires that can kill tin soldiers, it is located to the left of the ledge with the vortex in it.
You need to use the only supercharge ramp on the level and use it to jump the ledge with the vortex on it.