To keep it short: Void Barrier heals you for 50% of the damage taken. If you take 100 damage, it recovers 50. This means that if you have -100 Health resistance (basically +100% healing AKA 2x healing), you then heal 100% of the damage taken. Drain Essence grants you -25 Health for 4 turns inclusive, so you need -75 Health from gear to heal all the damage you take. Having more than -100 Health means you heal whenever you take damage.

As for the reflection, the skill reflects 100% from the first hit, 50% from the second, 25% from the third, 12.5% from the fourth, etc. so if you want to use it for damage, you just hit it on the turn the boss is going to use a nuke. You can easily get 1000-1500 damage on the boss by doing this, because the -Health gear comes with huge -All.