
The scariest part about Sense is her unbroken stealth
No mage can possibly hide the moment they cast a spell. Even Frieren gets detected by Laufen of all people, from long range, while trying to be discrete. However, Sense already cast her spell long ago, before even approaching you. Because of that, she can lunge from the shadows without breaking her radio silence. You absolutely NEED to be looking at her with your EYES. But even if you do that, you probably won't expect what's coming unless you have pre-knowledge of her specific toolkit.
Let me elaborate on the last point. When Fern attacks Frieren to reveal her weakness, and when Lernen shoots at her, she always has time to react even in close range. But if Sense maintained a normal looking hairstyle, it's theoretically possible for her to just walk up and stab Frieren in plain sight before she realizes they're fighting. Or maybe that's a stretch, as Frieren is built different. But I bet Sense could make it work against tonnes of other mages, even those with more power overall.
How often do fights begin from long range, anyway? They often END in long range, but very often, mages will close the distance initially. Fern and Frieren literally start most battles only a few meters behind Stark, and only move further back during the middle of combat. And that's against other warriors. Nothing suggests the likes of Denken would start blasting from a mile away if a mysterious mage approached him. Especially since he probably wouldn't even notice her in the first place.