RTF is a global modification that replaces all towns in the game.

As I understand it, immunity to earth magic doesn't grant immunity to the "slow" spell if it targets the ground – is this something that can be resolved?
For example, I took a simple approach: all creatures with immunity to earth (earth elementals and Lava Elementals) also received an ability that grants immunity to slow, but there are few individual abilities for specific spells, so this solution isn't really a solution...