So I have {platinum emperion} on the battlefield. My opponent attacks with {wurmcoil engine}. I just take it because my life total can't change. Does my opponent gain the 6 life?
I don't think so because damage causes loss of life, and I can't lose life, so it does no damage. Correct?
He still gains the life.
He is still dealing damage, but Platinum Emperion is stopping that damage from causing loss of life. This is why creatures can still die with the Empirion on the board - damage is still dealt.
Whether or not creatures die has absolutely nothing to do with {Platinum Emperion}'s ability though. :/
Damage is dealt but life is not changed. Lifelink only requires damage be dealt so they will gain life.