Having a bit of trouble with dual type DRs
So I have a lich here in front of my part that's supposed to have DR15/Bludgeoning AND Magic, but also immunity to cold and electricity. One of the players has a magic spear (piercing) and a magic shield(Bludge) that both have cold damage on them too. The spear when deals upwards of 25ish damage with only 1 point being cold, the DR is blocking it completely. It's also blocking the shield.
So is there a particular way I should be putting in these dual DRs to make them actually work?