To earn the "Skilled Warrior" trophy in Shadow of the Colossus, you must use a move that you probably wouldn't have otherwise bothered with due to its riskiness: the Downward Jump Stab.
Common sense tells you that when you reach a Colossus' weak point, you should hold on for dear life and plan your charged attacks carefully. To get this achievement you are going to have to throw that out of the window, by jumping (
) while standing on the weak point and then stabbing with (
). You must defeat
the Colossus with this attack, so you either need to make sure you know how many regular hits the Colossus can take before you deliver the killing blow with the Downward Jump Stab, or try and exclusively use Downward Jump Stabs for the duration of the battle.
Some Colossi are naturally excluded from this trophy because their final sigil is not on a horizontal surface - you can't jump down onto the weak point. Due to the trickiness of the trophy and the fact that some Colossi are far more suitable than others, my recommendation is to wait until you get to a New Game+ and head for the very first Colossus. Not only is the weak point on the flat point of his head, it's also a pretty slow beast and crucially has some nice long pauses between shaking animations where you can get your Downward Jump Stab to execute properly. The trophy should unlock during the Colossus' death sequence.