I don't think life leech is available on the mare and adding necro will not help. Life leech is only available on a few pets. Ossein Ram, Tsuki Wolf, and Saurosarus are the only ones I can name of the top of my head.
Keep in mind that life leech sounds much better than it really is. Every time a pet with LL is hit, LL fires and consumes 5 (I think) mana. All of those pets above have pretty crappy mana regens that are not fixable through training. So LL provides burst tanking up to the point the mana is gone which does not take long at all. While it's tanking effects are temporary, it's mana draining effect is pretty much constant while it's in combat (you may not have mana for AI, but there is always 5 points for LL every time a mongbat hits you). Which means that there is not a whole lot of mana left for other useful moves.