I don't have a lot of problems with the AI mainly because I've never played online and I'm used to do many things by myself.
When she gets in my way I just aim at her and sooner or later she'll get out of my way. Also I set attack reaction off so I can shoot through her.
Sheva's AI started getting irritating when I started playing on pro mode. She just isn't fast enough to ressurect me even when she is next to me! She preffers shooting instead! DUH!
However, I have to say that she is an excellent shooter. I once noticed her when she was aiming a spider and the laser of her weapon was magically locked on the target, even when the spider was side-jumping!
In cover mode I sometimes give her only a rifle as a weapon.
In attack mode a pistol and a rifle!
It's unlike that she will use it otherwise.