I use a sthil fs56c with a 3mm? squarecord which chews through them slowly at ground level without breaking the cord too often. being a new model it is pull cord but easy to start (never failed in 2 years) as you just pump the fuel afew times then set the choke to full on and once running the choke automatically cuts out. Good but fairly slow and expensive to buy.
I also have a BC 2145 brush cutter which is probably twice as powerfull and with a disc on makes short work of the toughest rushes and even chews through the mounds of purple moor grass. Apart from a tractor mounted unit I doubt there is anything better than the BC 2145 but it is heavy requiring a full harness. This machine has cut acres of rushes,brambles, and grass tumps and even chews through 1 to 2 inch gorse.
Your only other option is to spray.
Regen