Infantry Mage +10, infantry sword +2 or armor +1 for Arena?

infantry mage+10 because the game will make you encounter enemies with more merges in their units and more merges= more points

+10 mage, until you get the others up because merges are the most important thing to scoring, with Aether/Galeforce/Black Luna/Regnal Astra coming in a close second(if at least 2 team members have it)

+10 mage, the merges give you more points overall. Each merge gives you around +4 to their total “arena weight”. So, a mage +10 will have +40, while an armor +1 will only get +4.

TLDR; the mage will eclipse the other two in terms of arena weight.

Use fehbuilder, free android app, it can tell you the arena score of your units after adding in IVs, merges, and skills. I think there are some other units builders that can list the arena score but I haven't used others.