Who was the last strong wrestler he fought and beat ?? 
Ellenberger.
Ellenberger, and probably Rory, an argument could be made for DHK, as while not a wrestler, he is for the most part a grappler.
Inb4 people talk about shields shutting him down with wrestling 6 years ago. Having said that you can't compare other wrestlers to Gsp takedowns they're on another level. 
GSP has great takedowns, and yes it's both their toughest matchup to date
Go watch Rory vs condit. Gsp won't gas though