Basically, you need to have 1 or more friends, you kill the sentry, then both of you start to count down from 3 or whatever, when you reach the number your counting down to, use the crystal on the Mob, and you get 2 from 1 sentry, so if you have 5 people and time it right, you only have to kill one mob. :>
Don't know if this is an exploit/common sence but i hope it helps a few people.