Just gonna post some natures you can use for Magearna.
Offensive Natures:
Modest - Probably your best bet, great for a bulky attacking set/choice specs. (Probably your best bet if you are having trouble deciding)
Quiet - An alternative to modest in case you are running a trick room team.
Timid - If you're looking at shift gear sets, or choice scarf. (Not recommended since Magearna is slow as heck)
Defensive Natures:
Bold/Relaxed - Physical Defense option, Relaxed for trick room.
Calm/Sassy - Special Defense option, Sassy for trick room.
If you have no idea which one to pick I suggest modest (or quiet if you want to have the option of running it in a trick room team).
However, it depends on your teams and what they will need the most.
Remember that you can only get one so far in game! (Without trading/hacking)