1: I’d say go for the strongest one’s that you can deal considerable damage to. This mean either killing, or almost killing them with one flag. When considering opponents, also take into account the points they give, if there are two about equally difficult enemies go for the any that gives more points.
2: depends on your heroes, I’ll use myself as an example: I have the strongest bench in my alliance (leader of a training alliance), so I do not bother finishing up anything. I’ll go and soften up a strong team, then move to soften up a next strong team, as I have 6 somewhat strong teams (30 leveled heroes) that I can use the soften up strong teams. So basically use your strong team against strong team (hit-n-run if that’s what it requires), and with your weaker team go for finishers.
3: If you mean in one sitting: too much situation dependent, no right answer.
Or if you mean in one mouthful, as much as you can get in there and still chew somewhat properly.