Lifesteal Damage is simply damage dealt to the enemy along with HP recovered to the attacker proportional to the damage dealt to the enemy rounded-down. The skill of which uses Lifesteal Attack will always use the base element of the unit or enemy using it.
Lifesteal Damage attacks that deal 1 damage will recover 0 HP.
HP Recovered = Damage Dealt * Lifesteal Effect (%)
For example, Deadly Illusion Shida, whose Lifesteal Effect recovers 30-35% of damage dealt as HP, dealing 100,000 damage will recover any amount of HP between 30,000 and 35,000.