@Zerfius The answer to the arrow question is Speed 90% of the time. The other 10% is for certain tanks that you don't want taking turns (Baze and Sith Trooper are the two that come to mind). I run Protection on my Sith Trooper because the extra 10k protection will be more impactful than one or two more turns. Other tanks that have useful abilities (Old Ben, GK, Shoretrooper) need turns, so speed is better than surviving one or two more attacks.
The best general advice I could give would be to go to your arena shard rankings and look at the top teams to see how they mod certain characters. In general, this will give you a good idea what works and what doesn't.