Last season solar melee kill bounties didn't count celestial fire but it does for throwing knife. Getaway Artist arc souls don't count as ability kills even though it uses up my grenade. The secondary explosion in void melee kills don't count as ability kills and same with ball lighting. Warlocks are never going to win Guardian games because our abilities don't count as melee or grenade kills.
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Using a piece of equipment doesn't count as an ability, regardless of the 'ammunition' it uses for its kills. Secondary explosions also don't count, as it's no longer the ability itself doing the kill but a result of someone affected by the ability. (Titan melee also has this issue where the ability is still active but only the initial kill counts towards it).