Is the "Challenge Accepted" achievement broken?

Solution 1:

There are some challenges that don't appear until you've completed others:

Enemies:

• Little Person, Big Pain: Kill 10 Midgets
• Load and Lock: Kill 20 Loaders
• Bully the Bullies: Kill 25 Bullymongs
• Cruising for a Bruising: Kill 10 Bruisers
• Marauder? I Hardly Know 'Er: Kill 20 Marauders
• You Dirty Rat: Kill 10 Rats
• Mama's Boys: Kill 50 Psychos
• You (No)Mad, Bro?: Kill 10 Nomads
• Short-Chained: Shoot a chain to release midgets from shield
• Hurly Burly: Shoot 10 bullymong-tossed projectiles out of midair
• Just a Moment of Your Time...: Kill 10 surveyors
• WHY SO MICH HIRT?!: Kill 10 goliaths
• Paigineering: Kill 10 Hyperion personnel
• Constructor Destructor: Kill 5 constructors
• Another Bug Hunt: Kill 10 varkids (Unlocked with Tundra Express)
• Tentacle Obsession: Kill 10 threshers (Unlocked with Highlands - Outwash)
• Pest Control: Kill 10 spiderants (Unlocked with Frostburn Canyon)
• Crystals Are a Girl's Best Friend: Kill 10 crystalisks (Unlocked with The Fridge)
• Pod Pew Pew: Kill 10 Varkid pods before they hatch (Unlocked with Tundra Express)
• Die in the Friendly Skies: Kill 10 buzzards (Unlocked with The Dust)
• You're One Ugly Mother...: Kill 10 stalkers *(Unlocked with The Highlands - Outwash)
• Skags to Riches: Kill 10 skags (Unlocked with Three Horns - Divide)

Elemental:

• Slag-Licked: Deal 5000 bonus damage to Slagged enemies
• Boom.: Kill 20 enemies with explosive damage
• Cowering Inferno: Ignite 25 enemies
• Say 'Watt' Again: Deal 5000 electrocute damage
• Corroderate: Deal 2500 corrosion damage
• I Just Want to Set the World on Fire: Deal 5000 burn damage
• Acid Trip: Kill 20 enemies with corrode damage

Loot

• Open Pandora's Boxes: Open 50 lootable chests, lockers, and other objects
• The Call of Booty: Open 5 treasure chests
• Nothing Rymes with Orange: Loot or purchase 1 orange item
• Purple Reign: Loot or purchase 2 purple items
• I Like My Treasure Rare: Loot or purchase 5 blue items
• It's Not Easy Looting Green: Loot or purchase 20 green items
• Another Man's Treasure: Loot or purchase 50 white items
• Gun Runner: Pick up or purchase 10 weapons

Money & Trading:

• Psst, Hey Buddy...: Trade with 1 other player
• Whaddaya Buyin'?: Purchase 2 items with Eridium
• Limited-Time Offer: Buy 1 "Item of the Day"
• Wholesale: Sell 10 items to vending machines
• For the Hoard!: Save $10,000
• Dolla Dolla Bills, Y'all: Pickup $5,000 from cash drops

Vehicle:

• Passive Aggressive: Kill 1 enemy while riding as a passenger
• Turret Syndrome: Kill 10 enemies with a turret or vehicle mounted weapon
• Blue Sparks: Kill 5 enemies while power sliding
• Hit-and-Fun: Kill 5 enemies by ramming them with a vehicle
• ...One Van Leaves - Kill 5 vehicles while in a vehicle

Health & Recovery:

• I'll Just Help Myself: Get 5 "Second Winds" by killing an enemy
• This Is No Time for Lazy!: Revive a co-op partner 5 times
• Badass Bingo: Get 1 "Second Wind" by killing a badass enemy
• Heal Plz: Heal 1000 points worth of damage
• Death, Wind, and Fire: Get 1 "Second Wind" by killing an enemy with burn damage over time (Unlocked after Cowering Inferno level 1)
• Green Meanie: Get 1 "Second Wind" by killing an enemy with corrosive damage over time (Unlocked after completing Acid Trip level 1)
• I'm Back! Shocked?: Get 1 "Second Wind" by killing an enemy by electrocution damage over time (Unlocked after Say "Watt" Again level 1)

Grenades:

• Woah, Black Betty: Kill 10 enemies with Bouncing Betty grenades
• Health Vampire: Kill 10 enemies with Transfusion grenades
• Pull the Pin: Kill 10 enemies with grenades
• Chemical Sprayer: Kill 10 enemies with "Area of Effect" grenades
• EXPLOOOOOSIONS!: Kill 10 enemies with Mirv grenades
• Singled Out: Kill 10 enemies with Singularity grenades

Shields:

• Ammo Eater: Absorb 20 ammo with an Absorption shield
• Roid Rage: Kill 5 enemies while buffed by a "Maylay" shield
• Game of Thorns: Kill 5 enemies with reflected damage from a Spike shield
• Amp It Up: Kill 5 enemies while buffed by an Amplify shield
• Super Novas: Kill 5 enemies with a Nova burst shield

Rocket Launcher:

• Rocket and Roll: Kill 10 enemies with rocket launchers
• Gone with the Second Wind: Get 2 "Second Winds" with rocket launchers
• Sky Rockets in Flight...: Kill 25 enemies from long range with rocket launchers (Unlocked after Gone With The Second Wind Level 1)
• Shield Basher: Kill 5 shielded enemies with 1 rocket each *(Unlocked after Splish Splash level 1)
• Splish Splash - Kill 200 enemies with rocket launcher splash damage (Unlocked after Rocket And Roll level 1)
• Catch-a-Rocket: Kill 5 enemies with direct rocket hits (Unlocked after splish Splash level 5)

Sniper Rifle

• Longshot: Kill 20 enemies with sniper rifles
• Longshot Headshot: Get 25 critical hits with sniper rifles
• Leaf on the Second Wind: Get 2 "Second Winds" using sniper rifles
• Eviscerated: Kill 5 shielded enemies with 1 sniper rifle shot (Unlocked after Longshot level 1)
• No Scope, No Problem: Kill 5 enemies without using zoom (Unlocked after Longshot level 1)
• Snipe Hunting - Kill 10 enemies with critical hits using sniper rifles (Unlocked after Longshot Headshot level 1)
• Surprise! - Kill 5 unaware enemies with sniper rifles (Unlocked after No Scope, No Problem level 1)

Assault Rifle:

• Aggravated Assault: Kill 25 enemies with assault rifles
• From My Cold, Dead Hands: Get 5 "Second Winds" using assault rifles
• This is My Rifle...: Get 25 critical hits with assault rifles
• ...This is My Gun: Kill 10 enemies with critical hits using assault rifles (Unlocked after This Is My Rifle... level 1)
• Crouching Tiger, Hidden Assault Rifle: Kill 25 enemies with assault rifles while crouched (Unlocked after Aggravated Assault level 1)

SMG:

• Hail of Bullets: Kill 25 enemies with SMGs
• Constructive Criticism: Get 25 critical hits with SMGs
• High Rate of Ire: Kill 10 enemies with critical hits using SMGs
• More Like Submachine FUN: Get 2 "Second Winds" with SMGs (Unlocked after Constructive Criticism level 1)

Shotgun:

• Faceful of Buckshot: Get 50 critical hits with shotguns
• Shotgun!: Kill 25 enemies with shotguns
• Lock, Stock, and ...: Get 2 "Second Winds" with shotguns
• Shotgun Surgeon: Kill 10 enemies with critical hits using shotguns
• Open Wide!: Kill 750 enemies from point-blank range with shotguns
• Shotgun Sniper: Kill 10 enemies from long range with shotguns (Unlocked after open Wide! level 5)

Pistol:

• Hard Boiled: Get 2 "Second Winds" with pistols
• The Killer: Kill 25 enemies with pistols
• Deadeye: Get 25 critical hits with pistols
• Quickdraw: Kill 10 enemies shortly after entering ironsights (Unlocked after The Killer level 1)
• Pistolero: Kill 10 enemies with critical hits using pistols (Unlocked after Deadeye level 1)

Melee:

• Fisticuffs!: Kill 25 enemies with melee attacks
• A Squall of Violence: Kill 20 enemies with bladed guns (Unlocked after Fisticuffs! level 1)

General Combat:

• Knee-Deep in Brass: Fire 1000 rounds
• Afternoon Delight: Kill 50 enemies during the day
• ...To Pay the Bills: Kill 20 enemies while using your action skills
• Critical Acclaim: Kill 20 enemies with critical hits
• ...I got to Boogie: Kill 10 enemies at night
• Boomerbang: Kill 5 enemies with Tediore reloads
• Gun Slinger: Deal 5,000 damage with Tediore reloads
• Not Full of Monkeys: Kill 10 enemies with stationary barrels

Miscellaneous:

• JEEEEENKINSSSSSS!!!: Kill Jimmy Jenkins once
• Sidejacked: Complete 5 side missions
• Compl33tionist: Complete 10 optional mission objectives
• Haters Gonna Hate: Win 1 duel
• Yo Dawg I Herd You Like Challenges: Complete 5 challenges

In particular "Shotgun Sniper" and "Catch-a-Rocket" only unlock after completing other challenges to level 5.

Solution 2:

Some may think that the achievement is glitched, but in fact some challenges are unlocked through progress of other challenges, for example:

"Get 200 kills will rocket launcher splash damage" challenge will unlock the "Get 10 direct rocket kills" challenge.

The full list is available at Challenge Accepted Guide.