1.10 sorc

[Shade]

If 1.10 works the same way, you'd be looking at..

1. Apply lower resist and -target cold resist
2. Apply cold mastery

So.. you do 1000 damage to a monster of 180 cold resist, under 50% lower resist and 10% to target cold resist, and 50% cold mastery. Monster will have

1. 180-50-10=120%
2. 120*0.5=60% final cold resist

In 1.10, if you are dealing with immunities, all resistance-lowering skills have penalty of 0.2.
In case of your example
1. Apply Lower Resist and -target cold resist.
180-(50+10)*0.2=168
2. Apply cold mastery.
168*(1-0.5*0.2)=166.32
Monster is still immune.