Hi - I was advised to incorporate the following "catchphrases" into the letters: preeminent scholar, foremost/topmost in his/her field, the first to have done xyz, a leading expert, novel and groundbreaking research, international acclaim, among the very best researchers in xyz, seminal contributions.
I know how gratuitous these phrases sound - I was thoroughly embarressed, but most senior scientists already know that it takes such phraseolgy to get the job done. Good luck!