RE: focus when flying combat?
The focus for all guys just getting started with combat should be to see how long you can keep from getting your streamer yanked and only take "clean" opportunity shots when they present themselves to you. After awhile, you will start to recognize sequences of events that lead up to good, clean scoring opportunities.
Combat's no fun when everyone is picking up pieces off the ground, so get everyone on the same page to fly defensively at first. When we fly combat here, it is usually 1/2A [.049-.061] or .15 diesel at 35-40 mph. The only score we keep is who has drank the most beer. Most of our outings last all day with very little wreckage, sometimes none. A typical day is 10-12 matches that last the whole 15 minutes. So, we're talking 3 hours worth of dog fighting and the focus can drift a little. Each match is basically the time it takes to burn 2 ozs of fuel.
If the string leaders are made long enough, that obviously helps keep the target away from the plane, but most midairs are stupid, random T-Bone jobs that a long streamer can't help prevent.