for those who don't know about the 15 second rules, here it is !
If you fear, polymorph, stun or anything that makes the character lose control. it can only be 3 times in 15 seconds. after that he will be immune until you leave him be for 15 seconds.
that said, it means you can fear him once... he trinkets, you can fear twice for only half the time it normally would. he somehow get away again... you fear him a third time, the fear will last for only 2-3 seconds at max. because you feared him 3 times in less then 15 seconds, he will now be immune for the next 15 seconds.
so how can it be bypassed, simply said, you let him breath a few seconds between each ones. the thing reset every 15 seconds starting from the first fear. So if you fear, and he gets out, dont chain fear him right away give him like 2-3 seconds then fear again. that way he'll never be in the 15 second rules and thus will never become immune.
of course you can chain fear the whole group, the 15 second rules works for everyone, so let's say you fear the FC and then the healer, you feared two person and the 15 second rule wont be a problem.
besides that a warlock does its job and quite well, but you have to know this rule if you are a cc guy !