I doubt Erza can match Natsu's with flame empress armor. Even if she can better resist the heat, that does little in terms of keeping up with speed, power, or durability. In order to keep up with Natsu, she'll need armors like Nakagami to deal damage. Why does Erza need to have a specific armor to counter someone if she has can match their power, speed, or durability?Erza needs anti-ice armor against Gray just like she needs anti-flame amor against Natsu. You don't need an armor against sand cuz sand doesn't burn or freeze you. Sand has no internal effects, the sand will simply trap you or gash your skin.
Fire burns you, if you don't want to be a charcoal by the end of the fight, having some anti armor would be helpful.
Ice is extremely cold, if you don't want to have your blood frozen, some anti armor would be good to reduce its effects.
Smashing Slayer ice is no easy feat either. Compared to regular ice, slayer ice is durable.
There is a reason why slayer ice can instafreeze Atlas flame's flame. A regular ice Mage would have to put high concentration energy and tremendous effort to accomplish just freezing the base.
Besides, Gray can reduce the atmosphere's temperature low enough to make the weather conditions unfavorable to Erza. On the other hand, that same temperature will energize Gray.
I'm not saying the armor is made out of ice and will repell ice 100%, but she will need some sort of thick clothing to resist the freezing temperature.
Which of course will restrict her movement still and give Gray the advantage.
We've already seen people cut ice with far less than what Erza has in her arsenal.
