I did finally figure out why I was medaling so much in the warzones at first: I was hitting a lot of first-time achievements. Now that I've got a lot of those achievements already... I actually have to work for my medals.
It seems she still can medal quite a bit.
The overall damage over time she does isn't much, for the reasons listed above -- the strategy of staying the hell away from any attackers, and cast time. But when the casts get off, they really, really hit hard. But mostly, she benefitted in this instance from Rykem, who made her effectively invincible. I swear, two-thirds of those healing points all poured into Squirrelcorpse.
One comment my brother, the dog, made about Ancient Hypergate: "Don't bother defending the middle. If you get both pylons, then your opponent can't get any points." As the large number of Objectives achieved by my Zap Witch shows (not to mention the big victory), he has a point. That said, don't try to steal it right at the beginning. I've had opponents try doing that (and tried it myself), and it usually means nothing. It's more beneficial to go mid-game, when some of the opposition is in the respawn holding area and few are actually around to defend.
A Zap Witch also isn't going to cap anything if she doesn't have a healer nearby making her nearly indestructible, anyway.