name the trigger mytrigger. I'm assuming the sentries are in different groups (there's an easier condition if they're the same group, but no matter. this way works). Sentries named sentry1 - sentry6
Condition
this && ("alive _x" count [sentry1, sentry2, sentry3, sentry4, sentry5, sentry6] > 0) && time mod 60 < 15
activation
myflare = "Flare" camcreate [getpos mytrigger select 0, getpos mytrigger select 1, 108]