The phrase "concerted attack" means a planned and coordinated effort by a group of people to launch an attack or carry out a joint action. Full definition
There's evidence of concerted attacks on individuals. (politics.co.uk)
I soon found myself at the center of concerted attacks by those who believed, somewhat cynically (and quite illogically from a scientific viewpoint) that they could discredit the entire case for the reality and threat of human - caused climate change, if they could simply discredit my work and, indeed, me specifically (this is to be distinguished from the good - faith scientific debate and give - and - take, that is to be expected — and indeed is necessary, for the progress of science). (realclimate.org)
Apparently, BTG launched after surviving a «massive and concerted attack from its first moment of existence.» (hacked.com)