Legitimately never found an opportune time to attack a sentry bot's fusion cores. I've come close once or twice with a well-timed grenade but there's just never quite been a good window for me to run across the face of minigun fire to get to the back lmao
Yeah, that's the exact issue I have.
Either I'm just pinned down too much (I think most of the times I've fought Sentry Bots there's often been other enemies who don't have cool-down moments too, like Protectrons), or I will actually have managed to kill the Sentry Bot before it gets to its vulnerable stage.