Some things I thought of about the Link Arena

[Venusaur]

The first that came to mind was a Garmzerker, which doubles anything with less than 29 Speed. Ross would be preferable for maximum attack, while Dozla offers a more defensive option. Still, this means that Wyvern Knights are pretty much screwed.

The second was Ewan as a Super Pupil, he gets 32 SPD with Excalibur, while still having 29 Magic. Magic WTC isn't bad either, though Luna being nerfed hurts its viability for an inventory slot...

Last but not least, Eirika is interesting mainly because Sieglinde is the best sword in the game, not even including the fact that it frees up Audhulma for another to use.

I wonder what the best possible team would be?

[Icon of Sin]

A team I know of has Berserker Ross, Pupil Ewan, General Amelia, Warrior Garcia and Dozla, complete with all the supports in between. Typically, they should remain at level 1 since you don't want enemy Big Shields to activate.

[Albino]

My personal candidate for the best team (assume that all units are level 1 with all capped stats):

Berserker!Ross (Garm/Hammer/Halberd/Swordslayer/Hoplon Guard)

B

Warrior!Garcia (Brave Axe/Hammer/Halberd/Swordslayer/Brave Bow: S-rank Axes)

A

Ranger!Neimi (Audhulma/Brave Sword/Armorslayer/Brave Bow/Hoplon Guard)

B

General!Amelia (Vidofnir/Brave Sword/Brave Axe/Heavy Spear/Horseslayer)

B

Super Pupil!Ewan (Excalibur/Aura/Fenrir/Luna/Nosferatu)

A

Also, if you search for them on GameFAQS, you can find some topics on the subject that you might find useful or otherwise intersting, most recently this one.

[MacLovin]

GarmZerker

Garm+Warrior

Maybe Garm+GK, but with armour/horseslaying junk-its risky

Also, Necromancer!

With max con!