These are scattered everywhere and can last a few seconds or take a long time due to its complexity or frustration. However, there are a couple of them that last only a few seconds, and you can simply repeatedly play them until you hit 25 missions completed.
Simply pause the game, go to USER-GENERATED CONTENT->SEARCH->Infamous 2 plot summary->Play->Repeat. You can also search "XP Boost 2" which will have the same amount of time.
Remember to do 5 missions from the Newest filter option in order to get the Trail Blazer trophy. You also may want to add the "Auto-Collect 305 Shards" by user son-of-sam to get all of the blast shards in the game for the "Just One More" trophy.