Here is the full list of all 31 Square Heroes achievements.
Score Multi Kill
Win team game 4+ players with 75% or more of team score
Score two Kill Streaks in a match
Capture 15 Gnomes
Re-gen your health twice over
Win without anyone else scoring (3+ players)
Play entire round without dying (3+ players)
Score Double Kill
Score a Kill Streak
Score First Kill
Complete Tutorial
Survive Survival!
Kill with Proximity Mine
Kill with Laser and Machine in round
Host an Internet game with remote players
Host 20 Internet games with remote players
Score Mega Kill
Win 5 games in a row
Re-gen 200% Team health
Replenish 150% Team ammo
Win DM using only Explosives (3+ players)
Capture Gnome opponent almost captured
Score three Kill Streaks in a match
Complete 5 LAN games
Complete 100 games
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Kill with Cross-bow and Harpoon
Kill opponent with Nuke Bag
Kill with Fire! (Flamethrower and Molotov)
Earn all other Achievements
Score Devastation