Brush cutter as in a metal blade? Usually reserved for tougher weeds, bramble and scrub.
An 18v tool wouldn't really be up to that, the battery wouldn't last 5 minutes, you might burn the motor out, and if you tried to improvise a blade attachment, it might lead to injury.
I've used the DW cordless trimmer, excellent tool but not really up to much more than grass and light weeds, as it is designed to do.
When clearing the nettles behind our house, I always find that a hand scythe is much quicker than my petrol strimmer.