Nymph cast spells as 7th level druids, so the default spell list isn't completely set in stone. They can't spontaneously Summon Nature's Ally, but they can change their spells. Properly preparing the battlefield can increase the effective CR a little bit.
Spike Stones/
Spike Growth is typically excellent because while normally anyone can search for traps, Spike Stones specifically requires a 'rogue' to do so. Maybe Call Lightning to draw the PCs into the battleground. Aggressive Thundercloud (or greater) to use both actions to do some damage.
More assassin-like... Well, Blinding Beauty and Stunning Glance might enable some things. A
Rogue simple class template could be an interesting addition. 4d6 sneak attack, +4 Dex, Uncanny Dodge, Evasion(and it already has good saves). CR 8 though. Blinding and Stunning enables the Sneak Attack so maybe use the Druid spells as support. Like open with Entangle/Fog(with
Ashen Path) and Frostbite/Bone Fists/Vine Strike for damage, and Barkskin/Resinous Skin/Resist Energy/etc for buffs.
Another option would be
Swan Maiden, it feels a bit closer to what you are looking for, but doesn't do much more than a fighter with a couple of SLAs.
Dark Ice monster template makes it a bit of a better fighter. Notably, it gives it 2 claw attacks and 1d6 cold damage on attacks. However, since Swan Maiden is dex-based, the +str from the template doesn't help that much.
Advanced template would probably be a little better and doesn't have any drawbacks. It increases the DC of the SLAs too (by 2).
