If 2 monsters are destroyed and sent to your Graveyard at the same time, such as if your opponent controls one of your monsters and attacks another one of your monsters with the same ATK, you choose which one of those monsters you will look at when you activate the effect of “Machina Armored Unit.”
Judge Program Forum Rulings
These Ruling(s) have not been confirmed as applicable to the TCG.[Notes 1]
"Machina Armored Unit" is a Continuous Spell Card that has a trigger-like effect. This means that you can activate the effect once its condition has been triggered. The cards does not restrict the player from activating the effect only during his/her turn.
This effect can be used "Once per turn, when a Machine-Type monster is destroyed by battle and sent to your Graveyard". As long as the effect has not been used during the current turn, once the condition has been met, the owner will be able to activate the effect.
↑Konami Judge Program Rulings are considered unverified, as their source(s) are not publicly viewable. They are not confirmed as applicable to the TCG until they have been officially published by Konami.