Yeah you can click on a different target, but it won't stay on that specific target if the auto chooses to hit someone else. I've been just tapping over and over and over again on the same target I want killed, sometimes it slips to another target.
Some of the challenges like AGI Gear Challenge (healers and support) tier 2 requires other characters to die before the boss. It's the one where Savage Opress jumps up and smashes everyone's health away, and the floating Droids one hit your people out.