What it does
Leech Life is a Bug-type physical move. It deals damage and restores HP equal to 50% of the damage dealt. The recovery scales with whatever damage you actually deal — if you do 80 HP of damage, you recover 40. Hit a resisted target for 30 and you only get 15 back.
Bug-type hits Grass, Psychic, and Dark for super-effective damage. All three see play in the current meta, so the coverage is genuinely useful, not just STAB for its own sake.
Who runs it
Araquanid at 56%. Its primary weapon is Water-type, boosted by Water Bubble, but Leech Life provides Bug STAB coverage and passive recovery. In doubles — a format full of chip damage — being able to heal back some HP every turn you attack buys Araquanid turns it wouldn't otherwise have.
How to use it
Reach for Leech Life against Grass, Psychic, or Dark targets that your Water coverage doesn't threaten as hard. Those types often resist or are neutral to Water, but Bug hits them for ×2. The heal-on-hit means you're gaining ground even on turns when the target survives.
Remember the recovery is capped by actual damage dealt. If the target has 10 HP left and you deal 10, you recover 5 — not the full amount you would have done at full health.
Tips & strategy
Leech Life is single-target — no ×0.75 spread penalty in doubles. Full damage, full drain against whatever you point it at.
Grass, Bug, and Flying types resist it, so don't expect the drain payoff against those. Switch to Water coverage. Fire-types would take neutral damage, but Araquanid's Water attacks are usually better into Fire anyway because of the Water Bubble boost.
Magic Guard doesn't block the drain — Leech Life's recovery is a direct effect, not indirect damage. The drain goes through.