To pull this off, you'll need Mutons; I did not test this with Sectopods. At the top of the HUD is an indicator when you're focused on the Muton next to the health bar that shows what armor is remaining. Focus on hammering the enemy with sticky grenades. This should do some damage and knock off armor without outright killing the target. Once all of the indicators have gone dark, indicating the armor has been removed, unleash all hell on the monster until it's down. Take care to keep distance between you and the enemy, though, as Mutons do have a penchant for charging in to close-quarters combat. The grenades can easily take you out if you're too close when detonation occurs. (I died more than once to this.)
I have not yet tested it, but the Support class perk "Weaken" may assist with this. To be perfectly honest, I generally choose "Disrupt."
This one works nicely in conjunction with Don't Run, You'll Just Die Tired.