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