accidentally did this when i wanted to throw him out the window and somehow also got the door out the window as well

Actually that has its own achievement

or you can shut off everything and use door crashers

I got this by accident so after I got him to surrender i jumped him out of the window then i closed kicked the door to his office and it landed on him

The one for doorkicking him is different than the one for door slamming him. Also, there's one for jumping out the window with him.

Pretty sure I got this achievement by running into the door which sent it flying off the hinges into him with that one ability... forgot the name of it. The $8,000 one.

You don't actually have to shoot anyone to get Gessler's "I know you, you'll shoot" line. You simply have to have sufficient kills on record. I don't know the exact amount - I finished my first playthrough with ten kills (including Gessler), so having 9+ kills is obviously more than sufficient. Never shot anyone though - not until Gessler himself. I didn't even buy the Resolver until near the end of the game.

I got this achievement on accident XD

Another way to kill him is to make noise on the right side of the building (smash a window) then go up to his window with Deathfluke. He will shoot you but (after a few auto-save restores) you will survive and can put him at gunpoint that way. :D

I have no dialogue. I've killed him without he can talk and ... THE END!