Here is the full list of all 36 Wild Frontera achievements.
Kill 1000 enemies
Complete all Stages in normal or easy difficulty
Complete Stage VIII in any difficulty
Complete Stage VII in any difficulty
Complete Stage VI in any difficulty
Complete Stage V in any difficulty
Complete Stage IV in any difficulty
Complete Stage III in any difficulty
Kill Bufferatops in less than 1min 30 sec
Complete Stage II in any difficulty
Complete Stage I in any difficulty
Complete all Stages in MOST WANTED difficulty
Resurrect a friend in a co-op game
Complete a stage wihtout dying
Buy a Custom Quality Revolver
Buy a Custom Quality rifle
Buy a Custom Quality shotgun
Buy all the character upgrades
Find all the cards
Collect 4,751 gold
Do not let a single suicidal bandit explode while fighting SteelNugget
Kill Poncho Bob with 3 turrets up
Do not let more than one sniper alive at the same time
Activate all the detonators in less than two minutes
Stand on the last platform at the end of the encounter
Kill the sow while 5 piglets are alive
Destroy all the portals in less than 5 seconds
Kill the Executioner with a punch as final blow
Kill the BergMeister with explosives only
Kill 8 minions in less than 20 sec while fighting the mysterious masked man
Kill the Bufferatops ghost first
Save 100 damsels
Spare the life of 100 bandits
kill all the pigs before they flee near Pueblo del Norte
Kill all the Banditos running from Poncho Bob
Reach a 5 star wanted level in co-op freeroam. "Now that's hard", Bestio.