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They tend to arrive just after the wave when everyone quits :4_joy: . They tend to quit so they do not have to face the Hunters as it is usually a hard battle.
It has been quite a while but maybe wave 15. Thought there are two boss waves beforehand.
Update: Just had a quick go at Carrier and joined a team well in (round 14). After 1 round we hit the boss wave at round 15 and got Hunters (and Agents). We also got two more on level 17.