Attacks » Leech Life
If I attack with Venonat's Leech Life and do 10 damage, how many damage counters do I remove from Venonat?
You remove one damage counter, which equals 10 damage.
Source: WotC Chat (1999-09-30)
Attacks » *Attacks in General
When I attack, do I discard the Energy card(s) I used to power the attack?
Energy is not used up by an attack. Unless that attack specifically says to discard an Energy, it stays attached to the Pokémon.
Source: WotC Chat (1999-09-30)