What armor suits each element best? (I've never understood these things...)

The most important thing is matching up strength and intelligence to the right elements. If you can't get a full set of one kind and have to mix and match, at least make sure you're matching int/str properly.

Int-based elements: light, fire, shadow
Int-based equips: evoker, illusionist, arcanist; from the Grove, lightbringer, reaver, and pyromancer
Str-based elements: air, water, earth
Str-based equips: myrmidon, assassin, ravager; from the Grove, storm caller, tidal lord, and warden

But there are some preferences if you can manage to get full sets of the same kind. For example, fire and air are your damage dealers; they do best with equips that boost dexterity (evoker/pyromancer and myrmidon/storm caller, respectively). Shadow and earth are tanks, so they do best with more agility (illusionist/reaver and assassin/warden, respectively).

Thanks!

Wait, what's best for light?

Depends on what you want it to do. Often evoker is good, since light's healing is based on its damage output. It is possible to use light as a tank (e.g. I have a light-air-fire team), in which case you might choose illusionist for avoidance chance.

But in a typical balanced party (e.g. earth, light, air), probably go with evoker.

Okay! Thanks again!

If you want the best possible team, according to me you should have earth, water, and light. It is a little slower than my second favorite (Fire, Air, Light) but it has higher survivability so it can help a lot against some bosses. If you put the earth in the first slot, all you need to do is use its thorns defense (unlocked at level 30) which lowers the damage it takes while damage the opponent for a good amount, and then the ice barrier from you water type goes on your earth pet so it takes even less damage and regenerates. Then the light is there to just help out here and there.

Okay! Thanks!