Merlin isn’t a grand servant and his biggest feats so far at a basic level can easily oxidise Tiamat’s toxic Chaos Tide.

While Illya (Zwei) tanked Ea from an altered Gilgamesh by drawing magical energy from multiple dimensions.

Merlin is a grand servant

I mean Prisma Illya should definitely not be underestimated, she could definitely go toe to toe with servants. But even if Merlin isn’t a grand servant in this scenario, he still naturally qualifies as one and that’s very dangerous. Also stopping any of Tiamants attacks is no small feat, and he completely neutralised it. And that Darkened Gil definitely wasn’t as strong as normal Gil. I think she has a chance to beat some servants but Merlin? Unlikely in my opinion, although I love the creativity of this matchup.

Merlin is good at making getaways, he can't lose if you can't kill him

Personality wise? Merlin, if he is FIGHTING against someone not the just bullshiting, he's motivated, will cheat, trick and manipulate but without evil.

This is simply the nature of the careless cold inccubus, not saying Illya doesn't have a chance,the Kaleidstick and Cards are amazing items, but without some form of support she is just too... young and innocent to deal with this abomination

Merlin doesn't fight his own battles, he gets other people to fight for him and then escapes to a safe place and supports from a distance. Illya doesn't fight unless there is a reason for her to fight. This fight never happens, both because neither character would ever fight each other, and also because Merlin would do his best to never be anywhere near Illya to where she could fight him, and Merlin is REALLY good at being nowhere near people who want to kill him, just look at Caster Artoria who wants to decapitate him.