How competent Beric Dondarrion was, when Ned send him after Gregor? Marcher Lords are supposed to be good, but in AFfC Strongboar describes him very dismissively as “slight and callow”, do you think a better/more experienced commander wouldn’t have fallen for Lannister trap?

Strongboar’s kind of a dick who thinks everyone sucks compared to him, so I wouldn’t necessarily go by that. If you look at Beric Dondarrion’s record as a field commander, he does a hell of a lot of damage to the Lannisters with very few men, although he is somewhat reckless with his own life once he realizes that he’s immortal. 

Also, I think Strongboar’s a bit pissed off that Beric killed his uncle.