I bought one of these this year, to use with large tough-stemmed grasses:

It's a Black & Decker 20" hedge trimmer. $50 at Home Depot. It does a great job on thick-stemmed grasses like Miscanthus and Ravenna Grass. It's the first power tool I've used for grasses, and I wonder why I waited so long.

For less tough grasses, and all my Carex plants, I use Corona hand shears.

Fine-bladed clumpers like C. divulsa are a 2-hand job for me. One hand to hold up a handful of the blades, and the other to shear it off. I shape the clump into a ball, so it grows right back into a uniform clump.