strobe effect like a bomb

I assume you're referring to a big blast that then slowly fades out...?
Rather than messing with effects, I find it quicker and easier to make a two-step cuelist (step one: dimmer and rate @ full in 00 seconds, step two: dimmer fade to 0% in 2 seconds), and have the second cue set to follow through with a delay of about 0.5 seconds. If this needs to be a continuous effect, set both cues to follow, and the cuelist will keep running.