Single Stealers are Water Daimons and Elementals at early stages, while later the Elementals get to target multiple at the last two levels.
As such it is simple: Lifesteal cannot destroy a rit alone, simply because it only takes a maximum of 20%. The percentage is fixed from where it starts from depending which creature you use, however you can increase it by adding the Power Stat to it. 1 power which reaches the creature increases it by 1%. So if you have 6 creatures and six power they all get 1% more, if you have one creature with 6 power it gets 6%. (Given all are lifestealers and you use a 100% slider.)
If you think of stealing, once you have 100 energy on a creature against a lifestealer that hits, after one round it has 80, the second 64. So the next time less gets stolen and so on… Once you only have a few points, it cannot take any. That is why only lifestealers cannot break a ritual, since that requires 100% damage done.
Against high energy rituals lifesteal can be way more effective than plain damage. Even if you would deal 4000 damage per round, against 100.000 VE you’d steal 20.000 in one single round which get added to one of your own, random creatures.
Lifesteal, as such, is very important to keep your own ritual alive.
One reason why a single elemental is so strong at later stages is that it avoids being targeted by multiple and heals itself. Against high maximum vitality or high win counts or both it becomes useful, especially since you can input all your stats onto it, giving it a lot of defense.
This is only one way of using it, I hope this lecture has helped and you will find new, creative ways to apply it.