First of all, if it's a "surprise attack", then whoever is the attacker generally wins. In real life, a knife beats a machine gun or even a nuclear weapon if you can sneak up to your opponent by surprise.
Second, if it's a duel, then I suppose that if the Elantrian isn't allowed to pre-draw Aons then it doesn't look good for them. Mistborns/Ferunchemists do have the nifty trick of being able to access their powers almost instantly.
But, here's a quote from Elantris that illustrates why an Elantrian would probably win at least 90% of the time
"A burst of light shot from the Aon, the air warping and twisting around it.
The bolt took Dilaf in the chest and exploded, throwing the monk backward.
Dilaf crashed into the side of a building and collapsed to the ground. Then,
however, the priest groaned, stumbling back to his feet.
Raoden cursed. He dashed the short distance and grabbed Sarene. "Hold on." he
ordered, his free hand tracing another Aon. The designs Raoden crafted around
Aon Tia were complex, but his hand moved dexterously. He finished it just as
Dilaf's men reached them.
Sarene's body lurched, much as it had when Dilaf had brought them to Teod.
Light surrounded her. shaking and pulsing. A brief second later the world
returned. Sarene stumbled in confusion. falling against the familiar Teoish
cobblestones."
Several things are in this paragraph. First, Dilaf has been boosted with the sacrifices of fifty monks specifically to resist Aon magic. He's a magical superweapon. Yet, even though Raoden is so far from Elantris that he is only able to teleport about 50 feet, the basic power Aon blasts Dilaf off his feet and slams him into a building. A few pages earlier, one of the enemy priests is vaporized in a single hit from that power Aon in a square inside Elantris. I don't think a mistborn would fare any better.
Second, it only took Raoden a moment to draw a teleport Aon to get away. I'd say teleport pretty much trumps every single power in the Mistborn book series except for the gods in the last book. And that's all it takes to guarantee that an Elantrian wins a fight with a Mistborn. Step 1 is to teleport away, step 2 is to come back later, protected with a ton of buff Aons and a few dozen friends, and blast the Mistborn with targeted Aons from a safe distance.