Today I am mostly searching for the destruction manual for my new strimmer.... 
When I find it I have some 12 foot tall thistles to hack down in our shared 'green area'........ 
Sounds like you need a brush saw, Strimmers are more effort than they're worth.
(I can never find manuals either as Mrs K tidies them up, luckily most manuals are on the interweb now)