Start up the mission Blood and Rust. After you drop down at the end of the road you'll see some friendly titans, your goal here is to keep them alive through the onslaught of enemy titans and foot soldiers just to the end of the causeway. The best strategy I found was to get in front of them and draw the aggression of the enemies so the AI can't go and get itself killed. As long as you are pressing forward drawing fire and taking out the enemies you should build your Core meter up and easily dispatch the enemies. Fortunately this is at the start of the level so you can easily restart if they go and get killed.
Recommend you do this on Easy to make it a little less stressful.