The thing is, if a Necro can afford to spend 14 or 18 to do that, you probably don't have a good enough board or you ran out of attack spells. If you're worried about him killing off creatures, you could consider using nonliving creatures like skeleton knights and iron golems, or playing healing cards more liberally.
Usually, you should be playing pretty aggressively in that matchup such that the necro has to make a decision between gaining more dice or healing. Either warlord naturally has more hp and less channeling, so you should try to leverage that as soon as possible. With super-efficient heals for his creatures and really good lategame cards like Mort, Ziggurat, and the Altar, the Necro's lategame is pretty much better.
I would probably forgo using the Barracks in that matchup too, and stick to buffing your warlord and mostly summoning creatures that are level 3 or higher; the Necro's lategame stuff excels against lots of smaller creatures.
At least, this has been my experience; my losses as warlord in this matchup generally come down to the fact that I stopped pressuring the necro with meatier creatures and buffs.
