How do you get through to a Senior when they aren't the SME?

This is actually pretty common in the AF where the E4/E5 folks are truly the SME's as E7's and above are almost exclusive "management" and E6's are transitioning to that role. Like anything else, be polite, be respectful, and articulate why you are the SME (training, civilian school, prior experiance, etc) but temper that with understanding that you might not be seeing the bigger picture.

It's all in the timing and delivery of of how you communicate what you are trying to say, in this scenario.

When you really want to punish a superior that is wrong, the best way is to do exactly what they said to do. Naturally, this doesn't apply to anything that will result, even conceivably, in loss of life, limb, or eyesight, or is illegal or immoral. I'd also recommend saving this for someone that you have made a number of reasonable and respectful attempts to offer the better way to.

I have used the phrase, "Sir (or Sergeant), there may be a better way,..." as a respectful way to initiate that conversation many times. And I always grin when someone does it to me--which happens more and more these days.