Here is the full list of all 20 Fistful of Frags achievements. It takes around 15-20 hours to unlock all of the achievements on Windows.
Complete 100 games
Kick some enemies into water, fire or a large fall
Kill enemies ending their streak of 7 or more kills
Kill enemies with dual Colt Walkers
Kill some enemies while being under whiskey's effect
Complete 25 games
Drop 15 enemy player's hat by shooting over their heads
Get some frags while your health is lower than 15 HP
Kill some enemies severely injured by another player, while that player is still close to the victim
Kill a few Steam friends with bare hands
Your first kill in a game among other human players
Make headshots with bow from long range
Pick lit dynamite sticks thrown by other players
Get some kills with explosive chain reaction effect (their own dynamite kills them)
Get some kills shooting at a lit dynamite
Kill enemies while reaching 100% overweight due weapon accumulation
Complete 500 games
Complete the mobile cannon tutorial
Kill 25 enemies while sliding
Kill 25 enemies by kick while wall-jumping