The AI has two modes, Attack (Offensive) and Cover (Defensive). The default is Cover. If you switch it to Attack, Sheva will become much more aggressive and start at the top of her weapon list instead of the bottom. I haven't played with it on for long periods, so I cannot comment as to whether that would be a good idea or not. You make the switch by holding the "partner" button and selecting the mode with the D-pad.
I tried doing that today but it didn't seem to affect Sheva at all, it just changed my weapons.
On the PS3 version keep circle button, B button on 360 pressed...
Where your healthmeters are there should pop up 2 options for Sheva: attack
, use the d-pad to select one...