Let's say the armor is important, do we have to show let's say the guy wearing the helmet in one panel, then the legging and then the breast plate? Or we can shorten it even more? Usually, I would skip this scene, but only using 2 panels might be strange or too short. How would you make it as short as possible without making it seem out of the blue and confusing your readers?
A single large panel showing the character(s) putting on the armour would let you highlight any importance about the armour or putting it on without taking much "time" from a reading panel-by-panel perspective. That way, for someone reading quickly, the size of the panel can be the first thing that strikes to the importance of this scene without taking time.
As for highlighting it's importance then that comes in more through the lighting and general design of it (or how the character(s) are interacting with it/examining it.
If you've been creating the right expectations in your readers, then in most cases I don't see why you would need a scene to establish that characters are wearing armor. If people are going into battle, why would they not be wearing armor?
But, if you do need to establish upfront that the characters are wearing armor, you could do it with a single overview shot.
For example, if Princess invited everyone to the beach - a situation were you wouldn't expect people to wear armor - you could have a nice overview shot of everyone having arrived and looking at the ocean. Everyone is wearing swimwear, except Knighty McKnightface who is in full plate armor, because he doesn't understand the concept of going outside of the castle for anything other than battle. And no one commented on it until they got to the beach. Because it's funnier that way.