He wasn't necessarily afraid of her being killed by her in direct battle. Just in confronting her she'd slip away. Or have a trap set. Basically, however he fought her, he'd either only catch air, or he'd have to go into a trap.

Think of a porcupine in a hole. And you have a sword. Yeah, with a sword, the average human is easily going to kill a porcupine. But, either it'll slip away from you, or hole up and you have to go into it's hole.

I'd rather face a deadlier animal with a sword, like a wolf. Wolf deadlier than porcupine. But, porcupine might be harder to kill because of it's unwillingness to fight and going into a defensive hole.

...Porcupine might not have been the best example.

HOWEVER...

Lanfear. AKA Cyndane. Weaker as Cyndane. Enough to make a HUGE difference. No. Cyndane, who fought Alivia, who if not as strong as Graendal is as strong as Mesaana/Semirhage. With an angreal that made Egwene (or was it Elayne?) stronger than Nynaeave. And a host of other ter'angreal. Who nearly killed that Alivia with all those disadvantages.

Moghedian. Strong. Weakest of the Forsaken, but still a Forsaken. Very good with compulsion.

Graendal faced them both and had them both Compelled before Shaidar Haran stepped in. Compelling=defeating. That's pretty impressive.

And, Rand's scared of Saidin at that point. Will LT get it first? Can he kill her? Will he be hard enough to hurt a woman? Pre-Semirhage, he knew he wouldn't. She could have him compelled before he got a chance. Post-Semirhage, he knew he wouldn't hesitate.

So, in a straight up battle, not the most dangerous. But, still, incredibly fast compelling. I'd rate Lanfear as more dangerous but Graendal DID beat her.

But, there's only one way to fight Graendal before he just did the massive balefire: On her terms. Where she could flee or have all sorts of traps set.

I'd be more afraid to face, say, Mike Tyson in a fist fight than say...Kurt Angle (Olympic WRESTLER, but still a strong guy and tough. No boxer, but still very athletic and tough). But, if I went up against Kurt Angle knowing I'd be drug or he might be heading brass knuckles under his gloves, Mike Tyson's looking to be a better choice. And that's what Semirhage is. Strong enough to be worried about, but with the bonus of any fight with her being "rigged", more dangerous.