By meeting arbitrary conditions during gameplay, you will achieve these achievements.

Achievement	How to unlock
Absolute Domination	Perform comboes 100 times.
Aggression Apprentice	Invest 15 skill points in offense.
Alien Sushi	Defeat Ixthid.
Bad Timing	Have your attacks get blocked 100 times.
Bully Stomper	Defeat Horlan the Crusher.
Can't Always Win	Lose 10 battles.
Determination	Stand up 100 times after being knocked down.
Elder Corpse	Defeat Master Yee.
Face Smasher	Smash your opponent with a weapon 5,000 times.
First Blood	Hit an opponent for the first time.
Flawless Victory	Defeat Creatos.
Gladiator's Edge	Get a two-star rank five times.
Gladiator's Experience	Get a three-star rank five times.
Gladiator's Perfection	Get the perfect star rank five times.
Go to the Devil	Defeat the Archdevil Malzor.
Guts Wrecker	Kick your opponent 5,000 times.
Heavy Drinker	Drink 100 potions.
Jumping Jacks	Jump over 100 enemy attacks.
Master of Aggression	Unlock the Meteor skill.
Master of Passivity	Unlock the Crystalline Prison skill.
Master of Wizardry	Unlock the Rage of the Colossus skill.
Ninja Goner	Defeat Nagano.
Not Novice Any More	Finish the Novice difficulty.
Not So Charming	Defeat Amit the Charmer.
Not So Fast!	Dodge 1,000 enemy attacks.
Orc Destroyer	Defeat Tagon.
Passivity Apprentice	Invest 15 skill points in defense.
Professional Dog Trainer	Defeat Gargadan.
Relentless	Fight an opponent for 15 minutes in a single fight.
Sucker Punch	Counterattack successfully 1,000 times.
The Rich Get Richer	Earn 25,000 gold coins total.
The Winner Is	Win 50 battles.
True Gladiator	Play the game for 24 cumulative hours.
True Warrior	Finish the Warrior difficulty.
Try Harder!	Block 1,000 enemy attacks.
Unstoppable RAGE!	Finish the Gladiator difficulty.
Vensor Dispenser	Defeat Lord Vensor the 3rd.
What... Just... Happened?	Get stunned for the first time.
Winning Streak	Win five consecutive battles.
With the Fish	Defeat the Sea Witch.
With the Speed of Light	Get knocked down in 5 seconds.
Wizardry Apprentice	Invest 15 skill points in magic.