This is the easiest shot - point your 'aimer' directly at the enemy and use full power. I recommend using dragshot shooting mode for this, it takes less time than the other 2 modes.
Shotgunning can (and should) be done at any range, even across an entire map. One nice way to master shotgunning is to play on metamine and try to shotgun enemies from one of the slopes on the end. I have found some useful tips for shotgunning in this particular spot (the map is very popular). Credit to Talent1127 for sharing the information. You can find some good stuff on his guild's home page: (link's dead, sorry)
Ok: metamine shotgunning - Just use this visual aid created by Talent.
Some other shotgun pointers:
- As your shot travels, it naturally drops down from gravity. Even if there's wind pointing upward, it will still drop a bit as it travels across the screen. So for full screen shotguns you may want to use something like angle 6 or 7 to hit an enemy who is roughly even with you. When wind is against you, the drop effect is greater.
- If you have an easy shotgun, try to aim just below center (hit their eyeballs) ... a clean hit in their weak spot can do 275+ damage.
- If you can get true angle and shotgun, ALWAYS DO IT. Anyone who complains about this is a noob and a whiner.