This was added in the meantime.
In version OXCE 7.0.15 there two new operation for battle unit like:
```
u.getFire fire;
u.setFire 15;
```
Only probably problem is moment when unit catch fire, it could be applied by environment or by other scripts.
Because of this I think it should be handed by next turn scrip hook, where you will handle all units that have frost and fire at once
(stronger should win, and stay on unit, if have same strength then both could nullify each other).