Life Tap is actually a nerco curse skill, but wands and daggers can have "charges" (e.g. 92 charges of Lv 5 life tap) of this skill ((not +1/ +2/ +3 skill attribute) so that you can cast on the ubers for using non-necro characters. Of course, notably, draculs gloves and exile runeword shields also have chances to cast life tap on striking.
Personally, I use rare wands and daggers to cast life tap on the ubers on the first sight, followed by zealing them to keep casting life tap by wearing draculs glove.