An undispellable Immolate would be too good. That said they should take the tack they go with other classes. If you use X spell it prevents Y spell from being dispelled for Z seconds.
An Example:
IF you use searing pain your immolate cannot be dispelled for 10 seconds.
With that method you give us protection of immolate but leave in room for error that can be capitalized on by the defender. It also has the upside of allowing you to revitalize a spell that has gone out of use and give it a purpose.