Well, here's my design. It probably needs a little bit more explanation, though.
As you enter the room where Bowser is (or any other boss, I just used Bowser as the picture was already on my PC), he stands in a separate area of the room, sectioned off with lasers. He periodically throws bombs at the current place you are standing, and you need to move out of the way before they hit you. When the bombs hit a surface, there is a 3 second countdown, and then the bomb blows up, damaging you (if you are too close) and destroying a 2x2 block area from underneath the bomb.
The aim is to make an infitunnel above and below the bomb on the grey tiles, and then launch the bomb so it hits Bowser before it blows up. Bowser can also jump between the three 2x1 platforms every 10 seconds, to make it harder to hit him with the bombs. It will probably take 3-5 hits with the bombs to kill him, and after each hit, he reduces the time period between throwing bombs and him jumping between his platforms. You are also able to pick up the bombs up if you wish to move them to place in portals, throw into lava etc.
I guess, as a final boss, it is not particularly difficult. The only real difficulty is avoiding the explosions, and not falling into the lava below when too much of the floor has been taken away from underneath you.