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