Here is the full list of all 30 Headcrab Frenzy! achievements.
Shoot 500 headcrabs using firearms.
Survive 20 waves in "Random" game mode.
Don't be bitten at all during a single wave.
Survive 20 waves in "Ladder" game mode.
Perform 250 nice catches.
Beat the game on "Medium" skill level.
Beat the game on "Easy" skill level.
Survive all 8 "Ladder" waves.
Beat the game on "Hard" skill level.
Complete "HCF_Range" arena.
Complete "HCF_Factory" arena.
Complete "HCF_Labmoar" arena.
Complete "HCF_Temple" arena.
Complete "HCF_Dungeon" arena.
Complete "HCF_Killbox" arena.
Perform a 25x combo.
Cause some disorder in the Museum.
Win 50 matches.
Exterminate 1500 generic headcrabs.
Exterminate 2000 fast headcrabs.
Exterminate 2500 fire headcrabs.
Exterminate 3000 ice headcrabs.
Exterminate 3500 poison headcrabs.
Exterminate 4000 electric headcrabs.
Exterminate 4500 ethereal headcrabs.
Exterminate 5000 hallucinogenic headcrabs.
Miss 1000 times.
Spend 60 minutes in Frenzy mode.
Regenerate 100,000 health points.
Score 1,000,000 points in a single match.