Do liches have an innate ability to resist spells? They don't have magic resistance, but if I cast low-level ( fourth level and lower, I think) spells on them it just says "Spell ineffective". What is this?

i think they just have really high magic resis that cant be lowered or something.

Liches are immune to 5th lvl and lower spells

Im pretty sure breach does still work, but sunfire for instance, doesnt ...

<font color=Orange>How many breaches does it take to lower it a lot?</font>

breach only dispells the combat protections such as stoneskin or fireshield. You need the lower resistance spell to lower magic resistance.