Reach level 10 to unlock the Arena, where you can create a team and fight other players. The outcome of the battles are determined by statistics and not game skill.
Battle
You will know to block or evade when an exclamation mark appears on the enemy. Blocking reduces damage without wasting stamina. Evading will dodge an attack while still doing damage, at the cost of stamina. Once you have done enough damage, you will be able to execute a very powerful attack. Use it to finish your enemy as fast as possible.
Better ranks
Take your time during fights to get higher rankings. There is no penalty for taking your time to win. Build stamina and always have enough to evade attacks. To get a better rank, avoid getting damage as best you can. Avoid attacks that can be bypassed, and block special attacks. Getting the best possible rank will let you get more money, better items, and health regeneration. If your opponent uses a "raging" combo, block them instead of trying to evade three consecutive attacks.
Marketplace
Reach level 15 to unlock the Marketplace, which gives you instant energy and exploration refills twice a day at certain times.
Stamina
The combat system uses the stamina bar to determine which moves you can make. Stamina will constantly regenerate. Using the "Attack" option will deplete four notches of the stamina bar. Using the "Block" option uses no stamina. Using the "Evade" option will deplete notches of the stamina bar.
Upgrades
When upgrading proficiencies, certain abilities will require others to be at specfic levels. Focus on getting powerful abilities instead of just building a large number of them.