I'm fairly sure improved healing doesn't work on Drac. If it did & I were to cap out improved healing he'd be healing for 125% of damage done, which is ridiculous. It's my understanding that stats with a White value are static & unaffected by any bonus. (ie. Kraken's slow duration is also unaffected by Longer Disables)
To the OP, nobody can help you without some more information on the battle in question, but do bear in mind that Life Steal runes make a negligible difference to most heros at higher rarities.