The listed below Demons and Abilities damage doesn’t contribute towards Soul Leech despite the clear description of the tooltip of Soul Leech mechanics.
All single-target damage done by you and your minions grants you and your pet shadowy shields that absorb 3% of the damage dealt for 15 sec, up to 10% of maximum health.
Doom
Wild Imps - Fel Firebolt
Demonic Tyrant - Demonfire
Doomguard - Doom Bolt
Diabolist Hero talent - Mother of Chaos: Infernal bolt (Empowered Shadow bolt/Incenirate)
Diabolist Hero Talent - Pitlord: Felseeker
Soul Harvester Hero Talent - Demonic Soul
Soul Harvester Hero Talent - Soul Anathema
UPD. Hellcaster Hero talent - Wither