Best chance is to find high ground and initiate. If the paladin can't fly to it, he can only run. On even ground, the pally will just shield and walk towards you immune to everything. It's a really stupid concept that blizzard came up with. Nothing in this game should give you immunity to anything. Reduction? yes. Not immunity.
If you can blink far enough after he blows a shield, you might be able to burst him down. If he has engineering, you will die, no matter what.
You have to force them use the shield ability. Once that's done, you have a fair chance as a fire mage.
In the end, it depends on the player's reaction time. Some are really slow to realize their hp is depleting and just die before they can turn around. Take your chances :D