NPC spellcaster, spell rolls, and dispelling.

[Ediwir]

Morning everyone, I had an issue recently and realised I made a ruling on the spot but never discussed it afterwards.

Our guest of honour, today, is the lich, a lv12 spellcasting monster who has Dispel Magic prepared.
For ease and friendliness, we'll call her Lucy.
Lucy's Spell DC is 29, her spell attack is +22. She has +6 Int and is a lv12 spellcaster, but not proficiency is given.
Lucy wants to dispel the Fighter's Fly spell so that she can keep raining death and ruin on the party's head like a good lich should.

She then casts a lv3 Dispel Magic against a lv3 Fly, and needs to take a spell roll against the spell's DC (with a +0 modifier as they are the same level).

What's the spell roll bonus?

As a monster, she doesn't have one.
Is it the same as the spell attack, and so +22?
Is it her DC -10, and so +19?
Is it her level + int, and so +18?
Is it an arbitrary number that I feel like including, and so +20?

I might have missed a paragraph that explains what to do in this situation, but cutting player's legs is a staple of spellcasting monsters, and so I would love to really understand what I'm supposed to do in this situation.

[Edge93]

Spell Roll is always equal to Spell DC -10. Don't recall exactly where it's stated in the rules but the character sheets show this and also the section on scroll DCs and spell rolls mentions that the spell roll of an item is DC -10. I excpect it's stated more comprehensively somewhere but all indicators go to DC -10. It's like any other roll/DC combo too, Will Save/DC, Perception bonus/DC, etc.

[Ediwir]

That’s what I went with, but I cannot find a confirmation...