On every 5 waves done you unlock a gate giving you an additional troop.
There are in total 4 troops that you can unlock so by the end of level 20 you will get the achievement.
When a gate opens simply move your troop to the unlocked troop to collect him and join the fight.
After level 15 the game becomes a bit hard, so use your power ups wisely in order to survive the 20 waves and unlock all the troops and the achievement.
Good luck.