Here is the full list of all 15 Selknam Defense achievements.
You found a crude reality.
Beat the 7 minutes and 29 seconds on survival mode.
Raise all units to level 3.
You defeated the evil boss.
Discover the genuine evil.
Unblock the survival mode.
Sell a unit that is almost dead.
Confuse, stun and catch with a network the same enemy.
Reach at least eight buffed selknams at the same time.
Raise a complete class of Selknams to level 3 (archers, bowlers, warriors or mages).
Kill five enemies using only poison.
Pass one of the level from second world and above, on triple speed.
Switch position between two units.
Resist a suicide explosion with your Selknam tank preventing your allies death.
Complete the three first levels on campaign mode, obtaining nine stars.