The state of warframe forums in 2020...
A fight requires you to *stop shooting* for 3 seconds:
Forums: Unfair!1!! Bad Design!!!1! WAAAHHHH!!!
The reality is this, you are given a mech in the quest for Deimos for the explicit purpose that you can learn most of their mechanics. This includes their Iron Skin-ish ability. The mechanic is that later on when you are fighting mechs, you can easily and clearly(yes, its very obvious, if you can't, go visit an optometrist) see them use the same mechanic, all you have to do is stop shooting.