How can i best pull off splash and crash in infamous?
"Best" can mean several different things here. If you mean easiest then wait until you get Precision. It gives you much more time to aim during your fall. Set your camera option to "fastest" too.
If you mean more challenging then do it using only basic Lightning Bolt and basic Thunder Drop. Wait until the target is at a distance from you but coming toward you. Begin your drop, get one shot but avoid the head, activate thunder drop, then steer toward him. Be quick!
For even greater challenge, do it from street level, low object less than a story high, or low roof. I've never tried this from street level but I know that Thunder Drop works during a short jump like that. The damage radius is small, though, so you need to be at arm's length from the enemy. You might need to weaken it first with a body shot before the jump shot and drop.
The view camera is what makes any aiming stunt so difficult. You can't aim in advance before the drop and expect it to remain where you aimed it during the move, drop, or jump.