You can try for this over the two playthroughs you need to get the difficulty trophies, if you do not get it naturally there is more advice below as this weapon is a bit rare during the game.
You can go to the in-game bonuses buy the Wes-44 to instantly get it, this costs 500 points to buy, You can always save the game before buying the upgrade to reload after you get the trophy to get your points back.
Of course, I would prefer comments over negative votes, as it is way more constructive.