How to Unlock The Jock
To unlock the Jock, you have have to achieve Destruction Level 75 in a single level, and complete the level. This means that you have to destroy everything in a playthrough until you get the Achievement.
You could also choose the Scientist, and just use the Giantizer pills to wreck everything in your path. Mind you, these are especially effective in the Slums and Park Levels, but not so much in Industrial as most things are made out of metal.