Attack! Move!

Like all the quests from this particular daily hub, this quest is a reference to WC3. An attack-move action is, as the name suggests, a movement action where your units will attack any units they come upon. As the quest requires to kill weaker units, it matches what you use that particular move for in WC3, sending your own weak melee units against the enemy's without focusing fire.

The Raiders appear to have a very small aggro range or they take some time to attack...

I've found they respawn quickly - in fact, I sat in one place and continually killed the same one over and over until I had my 12 :)

There is a spawn point at 75.9 25.0 with instant respawns of Domination Point Raiders.

This way you don't have to pick the shamans out of the battlefield.