Duel
One on one battles. No special powers from feats or gear here, just skill against skill. You can fight against AI or Human opponents, but both are great ways to build skill without the other distractions of battle.
There is, of course, no one strategy that will win every duel, but here are a few tips that'll help you out.
Be Patient
You've got no points to cap, no allies to save, and no wandering enemies to put a blade in your back. So don't be so eager to execute the enemy in the first flurry- every class has abilities that work best after a parry or block, and an overhasty attack just leaves you open to devastating counter-attack. Be willing to play defensive at times, and chip away at your opponent's health rather than going for a single all-out push.
Don't Get Hung Up on the Environment
Human players see an arena with a ledge, sawblade, or water hazard as a chance for an easy kill, and tend to go a little overboard trying to snag it. If you continue to play it loose, being especially ready to counter any guardbreaks thrown your way, you can use their own impatience against them.
Charge
Almost every class has a special charge attack that only works outside guard mode. Humans know this and will watch for it, but bots seem particularly susceptible to such attacks. Use this to gain an early edge in AI encounters.
Disengage
This is de rigeur for quick fighters like the Peacekeepers, but any class can benefit from occasionally breaking free of combat, either to gain back some stamina, execute a special move, or just buy some breathing space. Don't be a jerk, making a human chase you all over the map for your last two hitpoints, but don't be afraid to go a little Errol Flynn either- dodging behind posts and jumping downstairs to gain a tactical advantage.