It's not only about raw strength.
To whit, wasn't Graendal drawing on a circle of compelled channelers? So who is to say how much strength she had.
Indeed, plus Graendal has her own plain gold ring angreal. Seems fairly even - she's facing three Forsaken-level channelers (Talaan, Alivia and Cadsuane + angreal) and two just under Forsaken-level (Amys is a match for Romanda/Lelaine, which isn't too far off Aviendha).
On a side note, when Perrin chased Graendal out of TAR did anyone notice that it seemed to read like the forkroot came out with Graendal as she was still spitting it out in the real world? I thought that things created in TAR could not be taken into the real world, otherwise the Forsaken would just pop in and have an unlimited supply of sa'angreal.
Weren't all the forsaken using the true power at the end, some in smaller amounts than others, but use of the true power would give Greandal and advantage as the weaves would be invisable to those she was fighting. Only a thought not sure if completely correct.