Here is the full list of all 34 OmniFootman achievements.
Defeat Undead Captain
Defeat Night Watcher
Defeat Royal Guard
Defeat Stone Devil
Defeat Deviant Spider Queen
Defeat Executioner
Defeat Valkyrie
Clear the sewer while equiping old sworld
Finish the game with "Challenger" activated
Unlock all weapons
Unlock all shields
Unlock all spells
Unlock all powers
Activate "Warrior"
Activate "Assassin"
Activate "Scholar"
Activate "Berserker"
Activate "Ghost"
Activate "Paladin"
Activate "Elementalist"
Activate "Knight"
Activate "Pyromancer"
Activate "Cryomancer"
Activate "ThunderCaller"
Activate "EliteFoorman"
Activate "Rogue"
Activate "Alchemist"
Activate "BattleMage"
Activate "Ranger"
Activate "Dragoon"
Activate "TheUnyielding"
Activate "Challenger"
Activate "WeaponMaster"
Deal 5000 damage in one shot