Tap Life is a spell that doubles your DPS damage for 15 minutes.
The cooldown on Tap Life, without Nanzuum Mana Cube, is 2 hours.
The ultimate depravity of the Arai harnessed to be used against them. This spell strains the very life force from all creatures within its range.
When you encounter Guardians that you don't quite have enough damage to destroy, Tap Life comes in great to double your damage and get past those difficult Guardians. If you have a Guardian encounter coming up shortly, you may want to hold off on using Tap Life to help make it past.
Tap Life can also be combined with Intensify to quadruple your damage for 15 minutes instead. This can help get by the toughest of Guardians. Though it is usually better to use Intensify with Dark Alchemy and buy permanent damage gains, it can be used with Tap Life to help advance in a pinch.