Go towards the wedding dance area. Following the sick drunk guest. Wait until he passes out in the burnt out building or help him pass out (by either sedating him or knocking him out). Take his gun and his wedding invitation.
Note: You can get his invitation and even his outfit without killing or sedating him. Apparently he really is that drunk.
Walk into the building and wait on the left hand side of the main stairs. When no one is looking, pick the lock on the door on the left hand side of the main stairs (middle door). When the guard reaches you (and no one is looking) sneak up on him and either sedate him or knock him out.
Note: Sedating this guard is easier than knocking him out, as you can move normally the last few steps you need to close the distance and sedate him while he is in the process of turning around.
Take his outfit and then go pick the door opposite the stairs (the bathroom). Go pick the other door in the bathroom. Wait for the guest in the bathroom to leave, then drag the gang member/guard through the bathroom and into the unused room that you just picked. Place his body in the closet. Retrace your steps and head up the stairs all the way to the attic. There are two winches in the attic; place a mine on the left one.
Head down the same set of stairs and pick the two doors near the stairs (one leads outside and the other leads to an 'L' shaped hallway). Go out side and knock the patrolling guard out. make sure the body can't be seen from the door you just came from.
Note: With the guard's uniform on, you can have the sixshooter out without the guards reacting.
Detonate the mine when the groom is playing the piano. Head into the L shaped hallway you unlocked and go into the room with the TV in it.
Note: The elephant gun is located on the wall in this room. It is recommended that you collect this gun at a time when you are not trying to get a Silent Assassin rating.
Wait in the closet for the father of the bride to enter and sit down on the chair (this will take a few minutes and a guard will patrol the room once in this time period). Sedate the father while he is sitting. Drag his body into the 'L' shaped hallway. Check to make sure the coast is clear and then drag him out side and throw him over the railing.
Note: You can simply throw him over the short railing in front of the atrium.
Go back downstairs and change into your suit. Go into the bathroom and then exit the building and head to where you started the level to exit and finish.
Alternative: Follow the father outside to the back of the property where he visits a grave, then simply push him into the grave.
Follow the drunk to the burnt out room the take his invitation. Now quickly run to the front door go inside to the main dinning room, then go out the right hand door and stand outside the window. Wait until the coast is clear and climb through the window and poison the cake. Quickly climb back through the window and run around to the left (make sure you go around the outside of the building and past the guard and the pillars you should see and disposal bin. Now this is the tricky part the grooms dad will walk out of a door up the stairs on his way to the grave. Quickly distract him by walking really close this should bide you enough time so the guard sits down and cannot see you. Make sure the guard is sitting down and poison the fat guy (just after the first pillar), drag his body to the bin and dispose of it, then run back to the start. Now wait for the groom to eat cake and die and the make your escape using the air boat.