I also find a lot of confused play in my games, where it's not clear if anyone has thought about working hard to win instead of working hard to make someone else win. Amending the Arena scoring could help. As well, gurus voting for a different format. But the best solution, one I work tirelessly toward and without any signs of success, is pointing out how player B is helping player A win. The most common response is silence, followed by "you should hurt A [so me helping isn't decisive]." I lack the social skills to adequately convey game theory, and just get blind-sided every time the second place player does their best to help the first place player, and refuses to see an alternative.
I've started mentioning that to short-circuit this erroneous play I'll just ruthlessly attack the second place player, but that seems not to have any effect other than grief. Oh well, if you can't beat'em, join them playing for second and attacking third.