The moment you guys opened a BATTLE ROYALE you gave the premise Elizabeth is willing to fight, so that should be out ofthe question.
Doesn't he actually "freeze" inside, meaning he can't move either?! So, in the end, do we get a locked up perma nuke?! Does that help anyone?!?
PC's hype is already dead, I'm wondering how fast Mel would take it down, then Elizabeth can show us how many dispel skills she has in her repertoire. Then it's down to stamina/MP lasting between the girls, although, from what we've seen of Elizabeth's current status and Merlin's experience in displaying her skills, one should think Merlin was the one with higher resistance, yet Elizabeth's been the one under nastier attacks, she even spent a while in a cocoon of Darkness(which should be far more dangerous to her) yet she gets herself up a lot faster than most with battle experience.
EDIT: I'm also wondering what Merlin could possibly do about getting locked up inside an Ark? Most of her skills are demonic, so if she tries any from within, I assume they get purged.
Willing to bet that current Ellie has inner self enchantments that should help her with defensive/resistance, the only question remains if Merlin would sneak behind her for a knock-out move and if that would work. I'm not talking about speed, but about resistance to (magic) attacks. We've come to see that even though Elizabeth gets toppled unlike King and Diane(sometimes), she's still the fastest of the rest in getting herself back up.
As for Mel VS Escanor, in a duo battle Ellie would most likely tell Mel about The One and Mel wouldn't be messing around...
EDIT 2: I just realised there might be a skill similar to Admonition in the GC, so, sumimasen, Escanor-chan, Elizabeth nee-sama is taking away your only toy for being naughty.
Which means they might change opponents. And Merlin clearly had no means of soloing AM Mel...
so, there's a reason why Melizabeth is the canon Power Couple of this series.