In Final Fantasy Tactics Advance, what will give a fighter the R-Ability Strikeback and where do I get it?

Answer 1

the armor that gives the fighter [and the gladiator] the reaction ability strikeback is called a Wygar, and looks sort of like a blue robe thing

when you get your clan's stat *track* up to level 30, you will receive a wygar as a free bonus

also, as you progress in the game, it will start to appear as a sort of common armor on enemies