There's an integer comparison: Unit - Level of ability for unit
If you want to use it as a part of calculation for damage, you will need to convert it to real. That is done via the option "Conversion - Convert integer to real"
Also, if you need more spells that target spell-immune units on one same unit-type, then your best bet may be to use the "Channel" ability and in the "Data - Options" field check "Universal" - that will enable that spell to be usable on spell-immune units.